Spirent TestCenter
DHCP release messages are always unicast because the client knows the IP address of the server.
Enabling Broadcast flag sets the broadcast bit to 1 to signal the DHCP server and BOOTP relay agent to broadcast any messages to the client on the client's subnet
From RFC2131 section 4.4.1, the DHCPRELEASE will be the flag will be set to 0, below screen shot for your reference.