I don't think I understand the question. FakePlayersMode can be set to either add or replace.
Both are valid. You can use any combination of FakePlayers or FP options with either add or replace.

Put simply (I hope), FPM=replace means that the values of FakePlayers and all FP options alone determine behavior. The number of human players connected is totally irrelevant when calculating wavesize and player-dependent HP modifiers in FPM=replace. In contrast, FPM=add sums the number of human players with each option value when determining the effective result, so the number of human players connected does matter.

Please consider these examples. I've chosen the values just for illustrative effect.


Example 1:


Results: Boss has HP for 3 players, Number of zeds in wave is for 4 players, Scrake HP is for 2 players.


Example 2:


Results: Boss has HP for 6 players, Number of zeds in wave is for 4 players, Scrake HP is for 1 player (FPM=replace forces zero-valued FakePlayers or FP options to be treated as though they were set to one, since zero is generally not meaningful for these parameters).

Thank, now i get every condition.

When I made my reply, I made a substantial copy-paste mistake in the "Results" under Example 1. I just edited it. Sorry for any confusion there.

Example 2 + Results were correct and did not change.

Should we have a Q&A thread with more details and more Question & Answer ?