If I am understanding your question right, I'll try to help.
Most AR platform rear BUIS, either fixed or flip-up are of a standard height, to match up with the height of a standard F marked FSB. The front BUIS will vary in height by mounting method, to match the height of a standard FSB/rear sight. A rail mounted front sight will have a lower overall height than one intended to be mounted on a low profile railed gas block (because the rail is higher). If a gas block with rail is designed to be the same height as the standard rail, you can use standard height front sights.
There are some rear sights for unique applications that are of a non-standard height. But, the info is usually noted in the descriptions.
My fear is that ahh, the whole island will ahh, become so overly populated, that it will tip over and ahh, and capsize...
Hank Johnson (D) GA YouTube