Yes, I think everyone is saying the right things in this thread, so I'm just going to parrot and try to say it in a different way. I think that anything that gets absorbed transdermally will have some small amount that goes systemic.
The formulation of the carrier is what is important as to whether or not it goes systemic of stays local. The trick is to first penetrate the skin - solvents and penetration enhancers will be necessary. But depending on which solvents and enhancers you choose, you will get different results. Some will deliver the product into areas that the bloodstream will pick up, while others can help prevent that to some degree.
With fat burners, it's a little easier (I think) because you just have to make sure it penetrates the skin, but not the layer of fat underneath. Fat tends to have lesser blood supply, so if you use certain carriers you will get the ingredients into the area where the fat cells are and it will site there for quite a while. Kind of like injecting into a muscle where it will go systemic pretty quickly vs. injecting subq into a fat layer where the release will be a little slower.
While some of the ingredient may likely still go systemic - there are plenty of user experiences that suggest localization works. Namely, a number of people have used clen in the Evomuse topicals and reported a dramatic decrease in clen-related side effects even at higher doses. Some of that decrease may be due to a lower absorption rate through the skin, but even large doses seem to avoid the side effects, so that wouldn't entirely account for the effects.
Some of the effect will also have to do with the ingredient you are delivering itself. Some ingredients will start causing effects the minute the touch a tissue - like ArA in Top Muscle. If you apply it to the muscle and can get it in there right when you work it, you will get a larger dose to the area being worked and it will have the effect locally before whatever is left over can go systemic.
I'm no expert and I hope I haven't mispoken here on the science - @dsade , @delsolrob and @nostrum420 are probably the top guys on this board for this info.