I have wondered that myself. I don't know, but I bet the virtual school will know the answer to that one. It must be important to many of their parents. If you find the answer, would you post back here with it?
By the way, I saw on your other thread that your child has no history of elective home education before the age of 16 but that they'd had home-based tuition provided by the school. You're right that this situation may not pass the Tax Credits litmus test to qualify for TC while being HE after Y11.Then again, maybe it does.
As I understand it, the reason for this requirement of HE before 16 is to try to weed out families who might try to claim TC when they aren't actually HEing their older teen. A previous track record of HE is one way to make the distinction. Possibly home-based tuition provided by the school could be seen as evidence of intent to home educate.
Interestingly, the critical question on the Child Benefit form does not refer specifically to ELECTIVE home education. The question is, "When did they first start to be educated somewhere other than at a school or college?" I don't remember whether there is a corresponding question on the TC form or whether qualifying for TC is contingent on qualifying for CB.
If you aren't familiar with the tremendously useful and comprehensive Edyourself website, have a look:
edyourself.org/articles/claimbenefitspost16s.php
Sorry to waffle. Hope that is a bit helpful.