The Relay Agent Information option(Option82) is inserted by the DHCP
relay agent when forwarding client-originated DHCP packets to a DHCP
server(RFC 3046). Servers recognizing the Relay Agent Information option may
use the information to implement IP address or other parameter assignment
policies.
The Dhcp Relay can forward the DHCP broadcast packets to a DHCP
server in a different subnet (RFC 1542). So DHCP server can provide
IP addresses to clients spanning multiple subnets insteading of deploying
a DHCP server on every subnet.