Re: When is the category of actual work/residence permit known
I believe that was not the question.
It looks to me that the decision is up to the cantonal authorities, and FOM just approves it. Judging from the application form in VD, the canton seems to select the type of permit, as well as whether it is deducted from the federal reserve or not.
There is also strong empirical evidence to back this up: At some places (e.g. ZH, TI, VS), as non-EU-EFTA, no way you're getting a B before two years on an L, even if you have an open-ended contract from day one. At some other places (VD, AG, SG) they give you a B immediately without L-ing you. If the decision were FOM's, I believe it would be uniform.