Question about Raspberry Pi 3 Model B

This means that it is not a standard mainboard (form factor), such as for PCs, or a socket for the CPU. The latter is permanently installed or soldered and cannot be replaced (onboard). The product is indicated in this way because it is listed in the same category as all the other PC mainboards, for example in ATX format. You can basically ignore the specification for the Rasperry Pi, this is more for the search, in case someone uses the filter in a product search.

See specifications:

Mainboard Features
Form Factor Special
Processor socket Onboard