Use SAVE when you are revising an existing document.

Use SAVE AS when you are creating a new document from scratch or an existing document. This will keep the original document untouched in its original format and create a new document with a new name.

use SAVE if you r saving the document for the first time. or if you r making any changes to the existing document.

use SAVE AS if you are making any changes to the existing document and want to save with the different name and with the different location.