I have interpreted this as "the frame on which the save requirement is met". If a replay is saved prior to this, it won't contain a completion frame (exception: if a replay that contained one was loaded, and is saved again without any modifications, before reaching the expected completion frame, it will retain the existing completion frame).
When playing back a replay in-game, the expected completion frame is not used in any way (but will be modified / erased accordingly if the expectation is not met). It is only used by the mass replay checker. The mass replay checker uses this to set a lower bound on the frame at which it will terminate and return an "undetermined" status - the previous behavior was "5 in-game minutes after the last action"; it is now "5 in-game minutes after either the last action or the expected completion frame, whichever is later". Note in particular that this will never cause a replay to terminate earlier than it already would have. However, it also doesn't override the existing condition of "terminate if no lemmings remain alive".