The usr_ tables are user designed objects. So this should be in your documentation…;-)
But you can use the SQL to find, in which module the usr_source object is.
Select * from md_database_table where md_name = 'usr_source'
Then you get the md_module_guid. With it, do the following:
Select md_title, md_name, * from md_module where md_guid = 'the guid from above'
My assumption is, this is linked in your config item object.
Hope it helps.