REG- SAP SD User exits for Available check

Dear all,

Can some please tell me or refer me to a link where I could get the details about User exits for Availability check.

User exits in program FV45VFZZ

USEREXIT_ADD_FIELD_TO_HEADER

USEREXIT_ADD_FIELD_TO_LINE

User exits in program FV45VFZY

USEREXIT_DELIVERY_GROUPS

USEREXIT_MVERF_INIT

USEREXIT_QUOTA_KEY_VALUE

User exits in program RV03VFZZ

USEREXIT_AVAILABILITY_IN

USEREXIT_AVAILABILITY_OUT

USEREXIT_DARA_REFRESH

USEREXIT_PLANT_SELECTION

I just want to know, what purpose each includes are used.

Thanks ina dvance,

Rgds,

Sivarajesh J