Precedent, as discussed in the previous two posts, is concerned with statements of the law. The Rules of Procedure of the Boards of Appeal refer in Article 21 to “an interpretation or explanation of the Convention”; in UK precedent, the emphasis is on the legal reasoning that is used to reach a particular decision and the exposition of law and legal tests applied along the way. The consideration of precedent is the degree to which a subsequent tribunal should use the same statements of the law, and the same legal tests in a similar manner, as used in previous cases. The important thing is that it does not refer to the outcome of the case, such as whether the patent is valid and whether it is infringed by a particular product.
Res judicata is about the outcome of a case, and refers to the principle that, once a matter has been decided as between two parties in a decision that is final (ie not open to further appeal), neither party can re-litigate the same matter again. This is not telling the Court how to decide a case; rather it is telling a party that it may not even raise the issue or seek to have it decided in a different manner.
The confusion between the two arises because in both cases one can use the term “binding”. In “binding precedent” a subsequent tribunal must follow an earlier statement of the law; when discussing res judicata one can say that the earlier decision is binding on the parties.
On reflection, it was perhaps misleading of the IPKat to refer to Article 112(3) EPC ("The decision of the Enlarged Board of Appeal referred to in paragraph 1(a) shall be binding on the Board of Appeal in respect of the appeal in question") and Article 111(2) EPC ("If the Board of Appeal remits the case for further prosecution to the department whose decision was appealed, that department shall be bound by the ratio decidendi of the Board of Appeal, in so far as the facts are the same") in the context of precedent in the original post, because really they refer to res judicata – the aspects of the case that have been decided by the (Enlarged) Board of Appeal are not open to challenge in the remitted proceedings. Thus if the Board of Appeal has decided that a claim set is novel, then in principle during the remitted case the novelty of those claims is not further open to question.
Res judicata is a relatively simple concept in principle, but its application in practice is actually rather complex, because its extent and applicability of the doctrine are frequently questioned. The recent UK Supreme Court case Virgin Atlantic Airway Ltd v Zodiac Seats UK Ltd  UKSC 46 (see IPKat here) concerned whether a finding of validity of a patent as between two parties was res judicata even if the patent was subsequently revoked. Overturning previous precedent, the Supreme Court decided that it was not.
What is particularly interesting from the point of view of the place of EPO decisions within European jurisprudence is that it is generally acknowledged that EPO decisions do not create any res judicata. Thus a person who unsuccessfully opposes a European patent at the EPO is quite free to challenge the patent again nationally, for example as a defence to an infringement action. For example, Grégoire Triet and Jean-Hyacinthe de Mitry of Gide Loyrette Nouel kindly drew the IPKat’s attention to a highly apposite decision, remarking as follows:
"We would like to draw your attention on the attached judgment handed down by the Court of appeal of Paris in 2005 [you can access it here or download it here].The same conclusion was reached in respect of England and Wales in Buehler AG v Chronos Richardson Limited (Merpel loves this decision because it cites the maxims "nemo debet bis vexari pro una et eadem causa" and "interest rei publicae ut finis sit litium" - there is nothing like a bit of judicial Latin). According to that decision, the same approach is taken in Germany, according to the decision of the Bundesgerichtshof in Zahnkranzfraser (Case No X ZR 29/93).
The background was rather classic. A patent owner brought an infringement case against an alleged infringer before French courts. In parallel, the infringer opposed the delivery of the patent before the EPO. The opposition was eventually dismissed and the infringement proceedings resumed in France.
Before the Court of Appeal, the patent owner argued that the EPO decision had res judicata over the Court and, consequently, that the defendant could not dispute the validity of the patent any more. The defendant disagreed and argued that EPO decision could not have res judicata over a French court.
The defendant invoked various grounds, the main ground being that an EPO decision originates from an administrative authority in charge of granting patents and not a court of justice handing down binding judgments. This was the ground adopted in first instance.
The Court of Appeal ruled in favour of the defendant and affirmed the first instance judgment. Although the reasoning of the Court is slightly different, for it found that (i) the preparatory works of the Munich Convention show that there was an intent to avoid that national courts be bound by EPO decisions, (ii) no section of the Munich convention provided that EPO decision had res judicata over national courts, and (iii) based on Section 22 of EU Regulation 44/2001, a national court has sole jurisdiction to revoke the national part of an European patent.
Unfortunately (for the sake of law of course), this case was not heard by the Supreme Court".
This gives rise to a situation that this Kat finds counter-intuitive, namely that a UK Court, for example, will give deference to the exposition of law in an EPO Board of Appeal case, but will then quite happily come to a different conclusion concerning, for example, novelty or inventive step. This can even in principle be on the basis of the same prior art, since a UK Court will generally take the view that differences in expert evidence , for example, may justify a different outcome. For example, as was stated in Generics (UK) Limited and others v H Lundbeck A/S  UKHL 12:
“National courts may reach different conclusions as to the evaluation of the evidence in the light of the relevant principles, but the principles themselves should be the same, stemming as they do from the EPC.”Even here, however, we see signs that UK courts are trying to be more consistent with the EPO, and in the HGS v Lilly litigation, reading the decision of the Supreme Court in Human Genome Sciences Inc. v Eli Lilly and Company  UKSC 51 (see IPKat here) it is possible discern in the judgment a desire to not only apply the jurisprudence of the EPO Boards of Appeal, but also to reach the same conclusion (of sufficiency of the claims) on the specific facts of the individual case.
If the EPO revokes a patent then that is the end of the matter and there is no patent for a national court to exercise any jurisdiction over. If the EPO maintains a patent in opposition, then, as stated above, anyone, including the unsuccessful opponent, can generally still challenge the validity of the patent in national proceedings. The degree to which a national court will defer to the EPO finding varies from country to country. Perhaps readers would care to comment with respect to their own jurisdiction how easy it is in practice to successfully challenge the validity of a patent already found valid by the EPO, such as whether it is generally necessary to use different prior art.