A user can have several profiles (a power user, or a member of a band), and a profile can have several users. So when a user creates an item, Jamroom needs to know which profile it belongs to.
That's the active profile: $_user['user_active_profile_id']. When a user views a profile they can change, it becomes their active profile. The + create buttons on that profile then create items on it.
create_save view, jrCore_db_create_item() uses the active profile unless you set _profile_id in its core values.Check that the user may change a profile with jrProfile_is_profile_owner($profile_id).