"We reserve the right to surprise", page-8

https://www.fda.gov/media/90014/download

A bunch of Q&A in that draft. I'm not one to be schooling anyone, but the way I understand it company will request voucher at the time they file for NDA, which I believe is "New Drug Approval". In order to file for new drug approval, they will have to have completed some clinical trials. Since they're not close to completing trial w/respect to DIPG, filing NDA for Pax is a ways off.
I noted from the Q&A section that PRV can't be used for a drug already approved for some other indication, so they'll have to do the work on DIPG and file NDA for Pax vs. DIPG before they file NDA for Pax vs. GBM (or anything else).

I could certainly be wrong about anything and everything, as it's about 3 a.m. here...