IBM 5150 Manual To Operations page 96

Write Protect Notch
Besides making sure your diskettes aren't scratched or
dirtied, you can make sure the computer cannot write
over information already on a diskette.
If your diskette has no write protect notch, it is already
write protected. This means that the computer cannot
store (write) any information on it. Your DOS Diskette
does not have a write protect notch.
If your diskette has a write protect notch, you can cover
this notch with a tab supplied with the diskette. Then
the computer cannot write on the diskette. In this case,
information can only be read from the diskette;
information already on the diskette cannot accidentally
be erased by being overwritten with new information.
Important diskettes (for example, purchased program
diskettes) are often protected this way.
VERY
IMPORTANT
DATA
Tab
Just for Reading
If you are certain it is okay to write on a write-protected
diskette, you can peel off the tab so that the computer
can write on it.
DOS 3-22