When a form is mapped to a process activity, Joget will create a record row using the PID as the primary key in the Application Form table. If you are using the Form Data Update Tool (Enterprise) to update the same Form, Joget would be able to locate the database table and matching the Primary Key (PK) using the PID.
Things get tricky if you are trying to update a Form that is NOT directly part of the flow. Joget will not be able to use the same PID to match the record row.
Figure 1: Transaction table which is mapped to the process activity.
Figure 2: Account table which is not part of the process activity but has relationship to the Transaction table.
Figure 3: Mapping of Form Data Update Tool for Account table.
The solution is simple. One will simply need to define a "id" field in the Update Fields section. State the value that would correspond to the record identifier of the Form.