The white expansion bus slot is the PCI slot, an older standard used for attaching peripheral devices to the motherboard. It is distinguishable by its color and was commonly used before being replaced by the more advanced PCIe slots.
The expansion bus slot that is typically white in color is the PCI (Peripheral Component Interconnect) slot. The PCI bus is a hardware interface used in computers to allow additional cards to be connected to the motherboard. These slots are easily identifiable by their white color compared to the brown of an AGP (Accelerated Graphics Port) slot or the various sizes and colors of PCIe (Peripheral Component Interconnect Express) slots, which have largely replaced both PCI and AGP slots in newer systems.
The PCI slots are designed for expansion cards that enhance the computer's capabilities, such as network cards, sound cards, or additional USB ports.
So, the white expansion bus slot is known as the PCI slot, an older standard for connecting peripherals to the motherboard which has been mostly superseded by the PCIe standard in modern computers.