It's got to do with the type of the case. Just like BMW and Volvo use numbers to signify the style of the car, Apple used them for the case.

Here are the ones they used for desktop Macs:

2: Compact Mac

4: LC Style Mac

5: AIO Mac

6: This one varies. Some are IIsi-size cases, others are low-slung Macs

7: IIci-size Mac

8: Minitower

9: Full size tower

Some 6 series Macs are actually in 7 series cases. Also, the Power Mac 4400 violates every rule.

For laptops:

1: Standard size laptop

2: Duo/small laptop

3: Standard size laptop

5: Full featured laptop