Dimensions can be assigned to master data then defaulted into transactions when that master data is used. For example, assign dimensions on a vendor that default into a purchase document or payment ledger entry. If you are posting from a payment journal and want a dimension to be posted to entries for the payment discount account (50350) it will most likely need to be entered manually on the line unless you can assign it as a default to the vendor.
Dimensions on payment journal lines will apply to all GL accounts used in the entry. Dimensions cannot be assigned to posting groups.
Example of Dimensions on Payment Journal Line. The Department and Area are both assigned as defaults on the Vendor card.

Example of Dimensions shown in Posting Preview (the third line in the entry is the payment discount account)

I am curious for the business case for tracking a department dimension on discounts taken GL account.