In the User Macros category you can choose a user macro for the current object class.
USR prefix, for example:
%[USR Activity List All]%
This category appears in the list when there is at least one user macro for the object class.
NOTE: If you want to see in details a user macro referenced by the placeholder, right-click the selected placeholder and choose Placeholder Definition. It opens the macro window. If necessary, update macro details and click OK. The placeholder would be updated with new values.
INFO: For details, see User Macros.