Yes, as Chris mentioned, the idea is that most of the times you’ll see only one field map correction method. In the case that both fieldmap and blip-up/down are available, my experience is that blip-up/blip-down methodologies seem to perform more reliably (and perhaps a bit better).
Finally, there is the card of the “IntendedFor” metadata field that you can use to assign correction methods to images (fMRIPrep will only use fieldmap information on images pointed at by this metadata field).
That all said, we of course welcome suggestions and understand that there are use cases where these heuristics are not particularly good. Happy to hear more from you (I think I can roughly guess where you are coming from )