You use custom fields to set up fields that aren't available on the Maintain Vendors window. For example, you can track vendors by location (in-state, out-of-state, international). Here in Vendor Defaults, you would enter the label as "Location". Then, when you're entering an actual vendor, you would enter the vendor's location in the Customizable Fields section of the General tab.
To set up custom field names for your vendor records,
Select the Enabled check box to enter or change the field label. If you decide not to use a field, simply clear the Enabled check box. The data for each vendor will remain, but it will be inaccessible. If you decide you want to use the field label again, select the Enabled check box, and the program will make the data available.