Derates are essentially a different engine rating, well, not really of course, but it is considered the same for the performance calculation. Which means you can legally use less thrust, but also take into account the advantages like, especially for contaminated runways, lower Vmcg/a, whereas Flex/assumed temperature cannot take that into account.
Have seen double digit V1 figures on the 737 with the use of derate on a contaminated runway (how would that work on airbus with the callouts?).
I am sure
john_tullamarine can explain it in much more (historical) detail.