Ah, now I feel like a right dim wit.
I added the rule to both the DiscType field and the DepPlanner field.
If DiscType=Commercial, then set field value DepPlanner=COMMInventory@xxx.com
That works!
Now, when I add another row to the table, the rule is not working or looping...it defaults back to select one...
Is there a rule that will make it repeat throughout the table?