Adding a Reference field on table A that points to table B, creates a relationship that ServiceNow can understand. This allows you to add a related list on records in table B, that displays the records from table A that are linked to it.

Creating a M2M table via the sys_m2m relationship definition table also creates a relationship that ServiceNow can understand. Because of this, you can add a related list on either of the two tables linked by the relationship, to show related records from the other table.

If you want to define a more complex relationship, however, ServiceNow allows you to do that as well! Let's explore an example of defining a custom relationship for tickets generated from.

Requests have a sort of task-hierarchy. The Request[sc_request] record is the parent of one of more Requested Items [sc_req_item] (commonly called RITMs), which may be the parent of one or more Catalog Tasks [sc_task], not to be confused with the base system Task [task] table...