Epson Apex 20020 - Manual (Page 243)

Write-protect
To protect the data on a diskette from being changed by
placing a write-protect tab over the notch on the side of a
5 ¼-inch diskette or by setting the write-protect switch on a
3 ½-inch diskette. When a diskette is write-protected, you
cannot erase, change, or record over its contents.
Glossary 17