The "Host IP" is the IP address assigned to the OS for the interface, and the "Redfish Service IP" is the IP address of the BMC for interacting with the Redfish Service.
For example, if "Host IP" is set to 192.168.1.10 and "Redfish Service IP" is set to 192.168.1.20, then I would expect that if you were to open a browser on the system and navigate to "https://192.168.1.20/redfish/v1/", you would get the Service Root resource on the Redfish Service.
Ultimately the physical transport really doesn't matter; TCP/IP is running on both sides, and as long as the system vendor has provided the appropriate drivers (or if they're using standard USB NIC devices that a generic OS can support natively), then from a user perspective, you just treat it like accessing any other network resource.
As tomchang mentioned, SMBIOS type 42 includes "HOST IP" (system/Host side) and "Redfish Service IP" (BMC side). I just wonder that which side Host/BMC will decide/provide these IP addresses? Per my understanding, BMC side (Redfish service) should provide its IP to Host using.
The decision about who assigns the addresses is really up to the implementation. As part of system initialization, I'd expect some sort of handshaking or occur between BIOS/UEFI and the management controller. As part of this, someone will allocate an IP address for the host and Redfish interface.