doctrine of res judicata are applicable to second actions involving substantially the same parties, the same subject matter, and cause or causes of action.53[53] In the instant case, there is no second action to speak of, involving as it is the very same action albeit the NLRC remanded it to the Labor Arbiter for further proceedings.   Principle of “Law of the Case” inapplicable   “Law of the case” has been defined as the opinion delivered on a former appeal—it is a term applied to an established rule that when an appellate court passes on a question and remands the case to the lower court for further proceedings, the question there settled becomes the law of the case upon subsequent appeal.54[54]   OSCI’s application of the law of the case principle to the instant case, as regards the remand of the case to the Labor Arbiter for clarificatory hearings, is misplaced. The only matter settled in the July 30, 1999 NLRC Decision, which can be regarded as law of the case, was the undisputed fact that Bastol was suffering from a heart ailment. As it is, the issue on the degree of disability of Bastol’s heart ailment and his entitlement to disability indemnity, as viewed by the NLRC through said decision, has yet to be resolved. Precisely, the NLRC 53[53] I Regalado, REMEDIAL LAW COMPENDIUM 472-473 (6th rev. ed.). 54[54] Meralco Industrial Engineering Services Corporation v. National Labor Relations Commission, G.R. No. 145402, March 14, 2008, 548 SCRA 315, 329-330.