Again, this is just a rule of thumb:
Jackets were rare in the post Civil War era.
(I don't know why.)
Sack coats were generally worn as a three piece suit, with only the top button, buttoned.
The length of a sack coat is generally covers the butt.
Frock coats were approxamately to the knee* and were worn with contrasting pants and vest. They were not buttoned at all, to show off a vancy vest.
Books
* I have seen short frock coats in paintings of the era. They were tailored like a frock coat but of sack coat length.