'Parent' field of the Task table is used as reference to map with other record. If for any reason, 'Parent' relations with other extended Task tables (eg. Request -->Parent , Request Item -->Parent, etc..) are not visible on the slush-bucket of Related Lists.
To get the 'Parent' relations with other extended Task tables on to the slush-bucket, update the dictionary entry of the 'Parent' column of Task table by pointing the Reference field under Reference specification section to 'Task'
In below image, slush-bucket Available section is not showing any relation with 'Parent'
Go to 'Parent' dictionary entry of 'Task' table and update 'Reference' field to 'Task'
'Parent' relations with other Task extended tables are visible to select on the Related list slush-bucket