When we flag as "Off-topic" and suggest another SE site, we can only choose from a couple options. I've been in the situation where I want to flag a post as Android or iDevice, but can't select these sites.
Is there a reason for this?
By default, regular users can only migrate questions to sites which questions occurring on Super User are likely to need to be migrated to. This is for several reasons, but primarily so that questions don't get migrated to the wrong place, and to reduce clutter in the UI.
Moderators (users with a diamond ♦ next to their name) have the ability to migrate questions to any site on the Stack Exchange network. If you think a question belongs on a site not shown in the standard migration dialog, flag it for moderator attention and leave a message stating which site it should be migrated to. Note that questions which are on-topic for another site but are also on-topic for Super User will not be migrated unless the person who asked the question specifically asks for it to be migrated.
If these come up a lot we can unlock the migration routes to Apple and Android.
However, all 5 migration slots are currently in use, so you would have to make a case that Apple and Android question migrations happen more than ...
You tell me -- which is more common?