the "backstrap" is the loin, it is on the outside of the ribcage. The tenderloin or "tenders" are 1/4-1/5 the size and on the inside of the ribcage.
If you go to your local Samsclub you can see both beef Loins and Tenderloins whole.
I have known many hunters to not even know they were in the body cavity and quarter an animal and strip the backtraps and not end up with any of the tenderloin. Shame.