SLIP accounts are for users who dial in to your system via a modem. PPP is more commonly used than SLIP for this purpose.
Most of the parameters for a SLIP account should be filled in the same as a regular login account. See login if you need more information on login name, full name, home directory, user ID, and the mail settings.
You can't set the group for a SLIP user; all SLIP accounts are in the
slipusers group.
The default command interpreter or login shell for a SLIP user is
/sbin/diplogin, which is a symbolic link to the dip utility for
handling dialup IP connections. If you have another script for handling SLIP
logins, you'll need to add it to linuxconf's choices using
Config--Networking--Users Accounts--Available SLIP Shells.