If you copy a large file. abort the copy.. but keep the broken file, and then copy the file again
And now you do resume and it continues at the position it left off (why copy the bytes you already copied before) like you do with resume download. Used mostly if you copy large drives images or other files that are many many GB large. and you abort the copy after say 50%. the you do not want to copy that 50% again.
Append if you want to join file.. it will add the source file to the target file. For example it is used when you archive a logfile and copy the active file to thr archive. then you append it so you got one large log file instead.