the difference is when you sweat the onions you want to cook without coloring, thats why this is the usual method for a white sauce or a light colored meal. but when you saute you slightly color the onions as cooking to add flavor

I wasn’t sure but now know by reading Kelly’s comment.

I think sautéing involves a little bit of browning where sweating just softens them without adding colour. Just gives a slightly different flavour and appearance I think

thanks sooo much for your perfect answer Kelly.

Sweating is the process of releasing flavours with moisture and low temperatures so the onion will retain a lot of its flavours and stay soft.
where sauteing uses a small amount of oil and its done in a uncovered pan using high heats so the cooking process occurs quickly and you lose some of the flavouring out with the oils.