If you want to configure mandatory dimensions on Sales Line, you cannot do this directly on Sales Line or on Sales & Receivables Setup. You need to do that on Item, Resource, G/L Account, Fixed Asset or Item Charge cards.
They will be transferred these dimension rules to the Sales Line. If Item has mandatory dimension, the Sales Line with this Item will have mandatory dimension. It doesn't matter is this global or additional dimension.
On Items, Resources, G/L Accounts, Fixed Assets or Item Charge, you will configure dimension as Code Mandatory, Same Code (need dimsnion value code) or No Code.