There is no out of seniority commands since SLS and COS08. It was agreed upon that there will be no early freighter commands, as well as that all new commands are only available in Hong Kong and not on a base. When you are assessed Cat A you will be able to bid on a base Command slot, otherwise you cannot. In case of the US, even if there will be a lot of command vacancies offered doesn't mean that pilots down the list with the right to live and work in the US automatically get the position and base awarded. These base slots have to stay vacant until someone with the right to live and work in the US, as well as being assessed CAT A (in order of seniority) bids on these slots.
Another bit of info most pilots are not aware off is this:
You are a based FO, Cat A'd and delay your command course date until you are able to hold a command base slot. You get the command base slot awarded, start your command training in Hong Kong and fail your command training before your 3 bar. Result - you go back to your original base as FO. You fail your command training after your 3 bar, result - you stay in Hong Kong as FO.