It means Dell has tested and verified that a 2.5-inch SATA SSD will work in the system. Actually, larger size 2.5-inch SATA SSD's have been shown to work, but that has not been tested and verified by Dell.

"each" can be 2TB

2 SATA AHCI 6 Gbps drives, 2 TB each for up to a total of 4 TB.

That is what Dell validated the machine with. You often see the 2 TB limit even on brand new machines. In my opinion it has to do with the partition style used. MBR partition style supports up to 2 TB, anything larger than 2 TB would have to be partitioned using GTP partition method. GTP can only be used when the BIOS supports UEFI firmware, which is any machine these days.

I am thinking the validation of  up to 2 TB comes from that, to provide backwards compatibility with older operating systems that cannot read GTP partitioned drives.

The Aurora R7 BIOS has among other features:

• UEFI BIOS SUPPORT to ASSURE Win10 Logo Compliance

So it will support GTP formatted storage drives and drives larger than 2 TB, although it was not officially validated as such.