I've got several ices on a remote server. On the top of them - rezzed Builder. I've got rezzed Wormhole, protecting another server (HQ, for example). The runner makes a run on HQ and faces the Wormhole. It's subroutine is triggered (resolve a subroutine on another piece of rezzed ice) and I decide to resolve the Builder's subroutine. On what server's ice can I put the advancement token? HQ or the server with the Builder on top?
If you use Wormhole's subroutine to resolve a subroutine on Builder, you place the advancement counter on a piece of ICE protecting the server Builder is on. Builder is resolving it's own subroutine, not Wormhole.
I've been convinced this answer is wrong, but I'm leaving it for posterity until I see an official ruling.
You place the token on an ice protecting HQ. The subroutine is being resolved on Wormhole, not Builder. I couldn't find an exact ruling on this question, but it's been ruled that the click ability on Bioroid Ice can't be used when Wormhole resolves one of their subroutines, and I think my answer here is a fair extension of that logic. Only if the ability referenced the Ice it lives on by name (e.g. Data Raven) should it not be referencing Wormhole (or in this case, the server Wormhole is protecting).