2 slots are used by thin SSDs and can be either SATA III or NVMe (PCIe). 2.5-inch SSDs connect to an available SATA III port. Here's all you need to know about them: Serial ATA (SATA): SATA has ...

A hardware interface for SATA drives using two lanes of PCI Express (PCIe) in order to increase the transfer rate over regular SATA. Motherboards came out in 2014, but few drives materialized.