Data Fields Manager allows you to select which contacts you want to modify. This is done via the Restrict option. Clicking on this option, you'll get the screen below:
First select the field you want to use as a restriction (in the case above, LastName). Then select what kind of comparison you want to do, and then enter the value to compare it to. The result of the above restriction, results in only 81 contacts now being affected by any mass field modifications you desire.
