SGMII support single 10M/100M/1G network port over 1,25Gbps SERDES between MAC and PHY, while QSGMII support 4 10M/100M/1G network ports over 5Gbps SERDES between MAC and PHY.
USGMII support 8 10M/100M/1G network ports over 10Gbps SERDES between MAC and PHY. This is most critical for high density switches and PHY. USGMII also provides packet control header to pass control/status between PHY and MAC. One of the application is Time-Stamp for PTP and 802.1AS.