Why can't I sort records using this field?

It is not possible to sort records using a field that is a nested table. Examples of such fields are Assigned To: (Task Information), Notify: (Commencement) and Notify: (Completion) on the Tasks tab:

The reason for this is simply that the sort value is not a single value.

Consider that for Task 1 (as listed in the Tasks table below), Notify: (Commencement) can have multiple values (Name 1, Name 2, Name 3); if a second task is added (Task 2 in the Tasks table), Notify: (Commencement) can have any number of other values (Name 1, Name 3, Name 4); and so on:

Which value is used for the sort?

For this reason, nested table fields are not listed in the Fields list on the Sort Properties box:

To identify whether a field is a nested table, use EMu's Field Help tool:

Place the cursor in the field.
Press the F1 key.
The Field Help window displays for the field. The Kind of the field is identified as Nested Table (and the column name has a _nesttab extension):

➤