The library macros could be said to come in three categories.

Primary groups which are always initialized. They provide basic library functionality and have minimal cost in overhead by always being available. They consist of things that are required to make the library function. Or, make the library fairly pointless without being present.

Secondary groups have some overhead even without using their functionality. They require initialization before there usage. Just by being available, they get initialized at program startup. If they are not needed, there are Conditional defines that can be used to prevent them from automaticlly being initialized when the program starts.

Tertiary groups require no initialization. There is no overhead for simply being available.