Changing usage tags via the API

Hi!
We are currently using the survey-tagging function for our reporting. (The dropdown field that appears when you publish a survey or the menu available under [Tools | Survey tagging] in the survey editor.)
Unfortunately, we now face the requirement to change a couple of hundred project labels, so my colleague asked whether we could do this using the API. But there seems to be no such field among the usual suspects, i.e., endpoints such as survey definitions or survey options.
Do I see this correctly or is it hidden somewhere else? Just want to make sure.