Great question!

So with AMEX GBT - Egencia - it routes back to Delta, and can be used by the passenger. That is, it shows on Delta, you can book it yourself, but Delta warns you and it shows "BUSINESS" in the (new) reservation.

HOWEVER, corporate OTA's are getting creative, and we now use Navan (formerly TripActions). In their case, you (I) can only book and redeem the value through their platform/OTA.
(and honestly I can't make sense of how much I had until they added a "airline credits" - of which I had a lot more than I realized)