What exactly does the spec “PCIe x16 slot ” mean?

Nvidia A2000 in SFF - Precision 3450 PCIe x4 slot 4x pcie slot Most of my gaming computer builds only use the primary PCIe slot with the 16 lanes and PCIe set in the BIOS I want Display Port 4x pcie slot In ASUS 「PCIe x16 Slots - 4」 - Motherboards Components , you could find most 4 x 5 x SlimSAS SATA 6Gbs 1-4

4x4 super slot I found things online like riser cards that seem to take one 16x lane and split them into 2 8x lanes, or 4 4x lanes, etc My thinking was if I