I Was Told the Best BBL is when the Fat is Only Harvested from the Back. Does It Make a Difference Where the Fat Comes from?

Hello dear!
Thanks for the question and provided information as well.
Fat can be harvest from many areas: arms, abdomen, back, flanks, thighs, etc. But overall, what gives you the best results is removing the fat from where it needs to be removed. 
In general, you need to be physically healthy and at a stable weight, have realistic expectations, be a non-smoker and your BMI needs to be 30 or less.
l recommend you to make an appointment with a board certified plastic surgeon to talk about your goals and anything you need to know.
Good luck :)

There may be different definitions of what is included in the Brazilian Butt Lift, but my experience, what gives the overall best results is removing the fat from where it needs to be removed.  I have performed this procedure for about 14 years now and have rarely come across a patient (who was a candidate for this procedure) that didn't need to have fat removed from the abdomen, waist and flanks (low back area).  I often remove fat from other areas as well, such as the bra strap rolls, inner or outer thighs as well as the upper arms.  The fat can be taken from anywhere, as long as the surgeon has a good technique and the patient follows the post-op instructions given to them by their surgeon.