I'm happy to help:

The most important ones:
F - Fax function
N - Network card (LAN connection)
D - Duplex printing
W - Wireless (WLAN)
MFP - multifunction printer (additionally e.g. copy function, scan function, etc.)

Other:
T - Tray (extra paper tray)
X - Network card, duplex printing & extra paper tray
M - Mailbox function (rather rare today)