As well as each hook returning the type appropriate to its interface, hooks have access to the "Vault object" (see the Vault Object).

The Vault object is exposed as the variable vault and is present during the execution of every hook.

This object provides an interface for retrieving required data, as well as generating transactions, triggering workflows, etc. (i.e. "hook directives").

Which hook directives are allowed for which hook are listed on the appropriate API versions' Hooks page.

Hook directives are not implemented inline; after the hook finishes executing, the bundle of desired hook directives is returned to Vault which will then attempt to commit them subject to various checks.