What a chosen model is for. Exhaustive rather than a free label: a
tag exists because some code asks for a model by it, so a tag with no
asker is a setting the person can fill in and never see used. It
grows when a caller appears, not when a name is imagined.
A tag rather than a store of its own, because “which endpoint and
which model answer this kind of job” already has a mechanism: a
person attaches an endpoint and chooses a model for a tag. A
second form and a second file would be a second answer to one
question, and the credential would need a second way into the
vault.
The model that reads text out of a picture: one a person
registered as able to read images. The city’s OCR tool asks for
it, and with none chosen that tool says so rather than handing a
picture to a model that only reads text (crates/kernel/spec/Model.lean §8-80).