From The RadioReference Wiki
WACN stands for Wide Area Communications Network. In Project 25, the System ID identifies the system (for a given network). One or more systems may be joined into a network of systems. The WACN identifies the network. The WACN combined with the System ID will uniquely identify a system.
The WACN is normally displayed as a five digit hexadecimal number (digits 0-9 and letters A-F) followed by the three digit System ID.