(HD) is Hard Drive ,(HDD) is a Hard DISK Drive It looks like this (HD) is a (SSD) a Solid State Drive. This is a (SFF) Small Frame Factor Design. The dimensions given  are for the shipping box. You won't get a BIOS Password or Windows 10 Key so get the keyfinder .exe from Microsoft store or you wont be able to repair most problems with this Referb. This will be my 3rd Dell computer the first 2 both had HDD's go bad within a few months so never get the HDD they are to fragile especially for shipping.