What the reasons for creating an .esm instead of an .esp?

[gooser]

I'm curious when it might be necessary for me to migrate my mod's esp to a be a master file. What are the pros and cons?

 

Thanks

[TheDriedFinger]

Masters have to always be near the top of a load order. The idea is that you only want your mod to be an .esm if it will be actually used as a Master (i.e. you or other modders will develop .esp plugins for it). Otherwise, it's a bad idea because you lose the flexibility of placing the files anywhere in your load order (except for where the .esm's are, of course).

[Guest corespore]

The vast majority of .esm filed mods are new land mods. The reason for this being that some in-game settings such as grass which are effected by .ini settings will not load correctly of the file is a .esp only and you usually end up with that annoying floating grass bug. For body mods and the such .esm files are usually overkill and in some cases a bad idea. However, for some body mods like hair mods that both the player and NPC's use a .esm is actually useful because it makes sure the NPC's hair and\or other body mods aren't effected by load order.