Build means compile and link only the source files that have changed since the last build, while Rebuild means compile and link all source files regardless of whether they changed or not. Build is the normal thing to do and is faster.

Thanks.

Thanks.  Suppose I change one copybook that is used by seven different programs, and then do "Build", Will it re-compile those seven programs?

Yes, it keeps a list of dependencies for each program including copybooks so if the timestamp on a copybook is newer than the last time it was built then it will build all programs that reference that copybook.