question about GenerateSpawnPoints option

when GenerateSpawnPoints is on, does it affect only when Action: add, or when only when Action: replace in the randomspawn config file for a mob?

I noticed for mobs where the Action: add, they will spawn in both daylight on night time, and also much more frequently when GenerateSpawnPoints is set to true.

but I think mobs with Action: replace spawn less frequently when GenerateSpawnPoints: true

There is no documentation I can find saying exactly what the GenerateSpawnPoints option does in...

question about GenerateSpawnPoints option