If you show your EAD as workpermit to employer, you "used EAD". For example, your H1 is expired, and you should show your workpermit, then EAD is only one you can show. You fill out I-9 with EAD, then you used EAD.
Or you change the employer, but you dicided not to transfer H1, then EAD should be the workpermit you should use. Again, you will fill out I9 with EAD.
Even though you did not fill out I-9 with EAD, if your H1 is not valid anymore but you are still working, it is implicit you are using EAD to work.