Fabric / backplane (ASIC)
Inside every switch is a single chip that shuffles packets between ports. Its total throughput is the hardest ceiling on what the switch can do — add up all the port speeds and if they exceed the chip, the extra ports are oversold. Cheaper switches share a smaller chip across more ports.
Detailed explanation
The fabric_capacity_gbps field — crossbar or shared-buffer bisection bandwidth. If sum(port_capacity_gbps) > fabric_capacity_gbps the switch is fabric-oversubscribed by design (typical of consumer/desktop SKUs). Datacenter SKUs run full non-blocking, so the only oversubscription you introduce is at the uplink.