Don't know of a formal definition but in common use:
A course is a length of individual cylinder used to make up a total length of a vessel or chamber of multichamber vessels.
Example: Vessel length between head seams 30', 3 ea. 10' courses.
I have not seen it applied to anything except the external pressure boundary.
Heat exchangers have 1 or more courses for the shell and one or more for the channel. On your U1 you will see "shells" treated in 6, tubes in 13, and "channels" in 14.
Hope this is helpful.