Under Art. 111(2) EPC the EPO department of first instance is bound by the ratio decidendi of the board of appeal if the case is remitted to the department whose decision was appealed in so far as the facts are the same. If the decision under appeal was taken by the Receiving Section, the Examining Division shall also be bound by the ratio decidendi of the board of appeal.
The ratio decidendi of a decision under Art. 111(2) EPC 1973 is the ground or the reason for making it - in other words, the point in a case which determines the outcome of the judgment (T 934/91, OJ 1994, 184). It is not contained in the order but in the section "Reasons for the Decision" and it is thus of no importance for its binding effect that a particular matter is not mentioned in the order (T 436/95).
"Res judicata" means a matter finally settled by a court of competent jurisdiction, rendering that matter conclusive as to the rights of the parties and their privies, such a final judgment constituting an absolute bar to a subsequent legal action involving the same claim, demand or cause of action, and the same parties or their privies (T 934/91, OJ 1994, 184).
The fact that the boards of appeal are courts was established in G 1/86 (OJ 1987, 447, point 14 of the Reasons, German and French versions - the board held that the English translation, "act as courts", was inaccurate).