If the Register DHCP Leases in DNS Resolver option is enabled, pfSense will automatically register any devices that specify a hostname when submitting a DNS request. The downside, of course, is that not all devices submit a hostname, and when they do, it is sometimes cryptic. For this reason, it is probably good practice to register important devices using DHCP static mappings, and other (unimportant/unknown) devices can be referenced using their IP addresses.