What’s the difference between MCIO, SlimSAS, OCuLink, and U.2/U.3?

MCIO, SlimSAS, and OCuLink are internal PCIe interconnects used to connect CPUs/host adapters/backplanes to NVMe devices or expanders. U.2/U.3 (SFF-8639) is the drive-side interface used by 2.5" NVMe/SAS/SATA SSDs. In short: MCIO/SlimSAS/OCuLink = host/backplane side; U.2/U.3 = drive side.