They are checking:
1. You are absorbing the vitamin D
2. You don't have any other conditions
I was told at my old surgery the lab would refuse to do the test if they tried to do it earlier than 6 months in adults. Children they have to check at 2-3 months because they are growing. The nurse who told me this seemed p*ssed off with the treatment protocol for the area, so the bone profile test request may be to get around this restriction.
When I said at my new surgery I needed a vitamin D test, the doctor saw that I had been severely deficient then wrote a form for a bone profile test.
As I have issues with iron levels I have private tests if I'm very tired as I've fed up of fighting with GPs. (Now you know why I know a lot about iron.) Anyway when I was anaemic last year and needed retesting after taking lots of iron supplements my GP offered to do a haemoglobin, ferritin and vitamin D test. There was no offer of a bone profile test.