This is a general warning for those who haven't tried FUE, and want very specific areas filled. I recently had a FUE procedure with about 70 hairs donated from a combination of beard and scalp hair. The purpose was to fill an irregular dime-sized thin patch on my beard under my right jawline. Some facial trauma years ago had damaged the follicles, though there was no obvious scar.
The first attempt left about half the desired fill area too empty and I had to go back soon after. But the second attempt still left a remaining third sparse. They mistakenly used too much hair to fill up slightly thin areas I didn't care about. It's also not clear if they got enough usable (non transected) follicles to cover the whole thing, but they claimed so. I assumed it would be obvious where to place the most graphs, but no such luck! I'll try it once again but will wait until the implants mature so I can be sure.
Based on my before/after photos, I reached several conclusions.
Make sure you show the surgeon (and "technicians" whom I found out do most of the graft insertion) a clear photo of what you want done. There can be misunderstandings about aesthetics and exact hair placement. It wasn't explained beforehand that the doctor would mainly do the graft extraction step. The two techs had varying levels of competency and it felt like interns were using my small case to get some training.
When the grafts are being inserted there may be enough blood or tumescence in the area to mask what really needs to be filled. I think this is what happened in my case. Coagulated blood spots may have been mistaken for inserted hairs, and when they scabbed over on the first round they got confused again on the second. Since you're not supposed to touch the area, it's easy to assume hair is underneath dark scabbed areas. I wasn't able to see enough detail myself after the first round.
Also, the eyesight (with or without magnifier glasses) of the surgeon should be confirmed as good. I didn't realize they were doing these transplants without extreme closeup vision. You can probably see more with a macro photo than they see through binocular glasses. The locations of graft site holes are critical, since the technicians may just follow where the doctor punched, even if they notice gaps; fear of the doctor's judgement, etc.
I'd let your hair/beard grow long enough for clear contrast. Don't leave it too short for any location vagueness. It looked clear in my macro photos but I didn't realize they'd lack adequate magnification on the chair. 5X lenses were mentioned; is that typical? Or was it a somewhat sloppy operation, I now wonder? They seemed professional enough and the doctor spoke like someone competent, with no red flags online. This wasn't Bosley or one of the other famous outfits. I am keeping details and photos private here.
If you have any hints that the doctor doesn't know exactly what you want filled, or is hastily deciding it for you, show them hi-res photos to confirm the graft sites. They may mean well but this is not a realm where poor communication can be easily forgiven later. It's your money and your hair! They may be too cavalier unless you insist otherwise.