The columns in the file are as follows.

INTERFACE - interface

Interface through which host(s) communicate with the firewall

HOST(S) (Optional) - [-|address-or-address-range[,address-or-address-range]...]

Comma-separated list of host and/or network addresses. If left empty or supplied as "-", 0.0.0.0/0 is assumed. If your kernel and iptables include iprange match support then IP address ranges are also permitted.