Save as saves a file that you are working on and will continue to work on.
Save a copy, saves a new file at that point and allows you to continue
to work on the current file.
Thanks for your response Bob, but it doesn't quite answer my query. I understand the "save" feature but it is the "save as" feature versus 'save a copy' feature that I'm confused about.
This will resave your file as a new file name - you continue working in the new file name.
Document A -
Continue working in Doucment B - Document A is in tact
Save a Copy
This will save your file to disk and you continue working on the original
File>Save as Copy
Name it Document B
Continue working on Doucment A - Document B is saved on the drive.
It's like a reverse of Save As