Customer screen > Account Details tab
Click the Catalogue Settlements button.
(Customer discounts must be enabled in system configuration otherwise the sub-heading and the following options are not displayed. When enabled you can set terms that if a customer pays in X days they receive an extra discount.)
Click the button to add entries for each catalogue.
Click Add to create a new row and pick the catalogue from the drop down box. Enter a discount percentage and enter the number of days that the settlement discount applies.
This allows you to set a settlement discount which applies only to one catalogue. Each catalogue can have their own settlement terms. Any entries specified at catalogue level will override the customer screen settings.
Click 
Update to save and close the window.