Basically, it's pretty simple. The caddies are constructed so that you can't put a drive in a non-compatible shelf. The numeric label is just to help you recognize the size. Sometimes they will have different colors (silver vs. black) for some speed differences between drives of the same size when those differences exist.
The "dongles" are just there to deal with missing "hot-swap" connectors on certain drive types, (P)ATA/SATA .