Asset Management enables you to get an overview of the assets in your network. An asset is a connected host in your network with an IP-address and is able to communicate with other hosts.
SecurityHive defines a host as an asset if at least one of the following criteria can be applied:
One or more open TCP/UDP ports are found
A hostname could be determined
An operating system could be identified
An asset always has a unique IPv4- or IPv6 address.
Too many assets showing up
Do you see assets that you didn't expect? Your firewall may respond to SIP (5060 / 5061) or SSCP (2000) ports for VoIP ALG inspection. this causes your firewall to respond to these ports and, in turn, the assets to show up in Asset Management.
Read more about this issue and how to solve it: https://community.fortinet.com/t5/FortiGate/Technical-Tip-FortiGate-is-not-forwarding-TCP-ports-5060-5061/ta-p/193853?externalID=FD36152