Hi Richard,
I never used it but there is a feature for it: " discount (Optional) – Integer/Float/Boolean – Discount offered? Default to false . Otherwise, you should include a numeric value."
source: https://matomo.org/faq/reports/advanced-manually-tracking-ecommerce-actions-in-matomo/
So, no need to send a custom dimension.
But what I can see is that there is no dimension for it within the segment feature, so if you would like to create a segment to get people who are using a discount, this feature would need to be created in Github, done: https://github.com/matomo-org/matomo/issues/19661
Have a great day,