The answer is...both. when you 'lift' the bark, some of the cambium cells stay on the underlying sapwood, others with the 'backside' of the bark.
You can improve your success rate even more by using your grafting knife to 'shave' a little bit of bark off the sides of your T-bud piece to expose a little cambium on the side - I do this with all of my bark grafts, which are, for all intents & principles, a very similar procedure.

lucky_p
Can you explain "to 'shave' a little bit of bark off the sides of your T-bud piece to expose a little cambium on the side -" so that it is easy to visualize.".
Do you mean that instead of verticle cuts on the T-bud bark, the edges of the T-bud bark should be thinned by shaving top layer to get closer to cambium layer so that it is touched by the cambium layer of the covering bark of the branch? That will require some delicate surgery!