This function is primarily used in conjunction with:
Landing Pages
Leads coming into the system by email
Custom web forms that allow for the automatic assignment of the Account Manager and/or Partner Rep by ZIP3 or ZIP 5.
Basic page:

Logic level can be set up and ZIP3s and/or ZIP5s can be assigned to them. The Logic level is then built into and referenced by the custom web form so that when records are add to a WorkGroup they will be automatically assigned.
Add Record allows for the manual input and maintenance of ZIP5/ZIP3 assignments:

When custom web forms are developed, the user usually provides a file of their Account Managers and/or Partner Reps by ZIP5 or ZIP3 for loading into the above table.
Here is what the Zip Code Assignments page would look like after a ZIP3 was set up:
The Default setting is used to designate a default Account Manager and/or Partner Rep assignment if there is no ZIP5/ZIP3 match in the Zip Code Assignments table.