When a scoped application table extends off of another table (i.e. task) the fields from that extended table are not available to be selected on the UI Policy Actions when creating a UI policy on that scope application table.
The user who is creating the UI Policy is not on the same application scope (i.e. on global scope instead) as the scoped application table.
Switch to the same application scope as the table's application scope.
For example, the table "x_abc" has the application scoped "Test ABC". Switch to that "Test ABC" application scope prior to creating the UI policy to be able to access the fields from the extended table (i.e. task).