Epson AcuLaser C8600 Reference Manual page 210

Note:
The 250-Sheet Paper Cassette A3W cannot be used together with the
standard lower cassette.
1. Pull out the standard lower cassette and remove it by lifting
it up.
2. Put both legs of the optional 250-Sheet Paper Cassette A3W
onto the printer's guide rails, then push the cassette all the
way in.
.
208 Using Options