Libraries hold custom functions for your application, which can supplant and extend standard functions. For example, the OperatorHasPrivilege function verifies that a user has a specific privilege.
However, you cannot create libraries in branch rulesets. Libraries are based on the ruleset name only, not the ruleset version. Because you must merge branches into a particular ruleset version, you cannot create libraries directly in branch rulesets.
To use libraries and functions in branches, perform the following steps:
Create a library and save it in the base ruleset.
Create the functions that the library contains and save them into a branch ruleset.
Merge the branch so that libraries and functions are contained within the branch.