+RT uses a convention to denote the foreign key status in its tables.
+The rule of thumb is:
+
+=over 4
+
+=item When it references to another table, always use the table name
+
+For example, the C<Template> field in the C<Scrips> table refers to
+the C<Id> of the same-named C<Template> table.
+
+=item Otherwise, always use the C<Id> suffix
+
+For example, the C<ObjectId> field in the C<ACL> table can refer
+to any object, so it has the C<Id> suffix.
+
+=back
+
+There are some legacy fields that did not follow this rule, namely
+C<ACL.PrincipalId>, C<GroupMembers.GroupId> and C<Attachments.TransactionId>,
+but new tables are expected to be consistent.
+
+
+=head1 EXTENDING RT CLASSES
+
+=head2 The Overlay mechanism
+
+RT's classes allow "overlay" methods to be placed into files named Filename_Vendor.pm and Filename_Local.pm
+_Vendor is for 3rd-party vendor add-ons, while _Local is for site-local customizations.
+
+These overlay files can contain new subs or subs to replace existing subs in this module.
+
+Each of these files should begin with the line
+
+ no warnings qw(redefine);
+
+so that perl does not kick and scream when you redefine a subroutine or variable in your overlay.