Feed Rate Modifier relative or cumulative?

First of all, Octoprint is awesome.

Is the value I enter in the Feed Rate Modifier always relative to the original value set in Cura, or is it relative to the last value entered in Octoprint? In other words, if I start at 10mm/s and enter 200% I should be at 20mm/s. If I enter a value a second time, this time 300%, am I now at 30mm/s or 60mm/s?

Thanks!

It is "relative" in your terms.

If you start at 10mm/s, and enter 200% you would get 20mm/s.
If you then enter 100%, you are back at 10mm/s.
If you want 30mm/s, you would need to enter 300%, regardless of what you entered before.