A good guide to how it all works is A Beginner's Guide to Trackback (opens in new window) by Mena and Ben Trott, the original designers of the Trackback specificaion.
-Trackback pings are stored as general-comments. Because general_comment is anot a content repostiory content_type, trackback is not a seperate object_type. This will be fixed in a future version. Additional information about the weblog that sent the ping is stored in the trackback_pings table. +Trackback pings are stored as general-comments. Because general_comment is not a content repostiory content_type, trackback is not a separate object_type. This will be fixed in a future version. Additional information about the weblog that sent the ping is stored in the trackback_pings table.
Currently any object_id that allows general_comment_create for the "Unregistered Vistor" user (user_id 0) can receive a trackback ping. That is the only activity required to allow trackback to be sent to an object.