The Custom tab can be found by selecting the ‘+’ Expand icon in the tabs row
The Custom tab allows you to enter data specific to your Brokerage such as PID/Roll Number, Registration Number, etc. to populate reports/documents such as the Other Trade Record Sheet (Other), Conveyancer Instruction report, Conveyance Letters to Buyer Broker, Listing Broker and Sellers Attorney etc.
The User Defined Fields panel is designed to house specific data you would like to track. This information is available on the Transactions by User Defined report available in Reports > Transaction Reports