The first digit tells the form factor: 6xxx is the thin form factor as in the Quadra 610; 7xxx, the taller form as in the Quadra 650 (but the number derives from the Quadra 700); and 8xxx, the mini-tower form as in the Quadra 800.
The second digit indicates the generation of Power Macintosh computer.
The last two digits signify any particular market designation.
For example, the Workgroup Server 9150 uses the 9 series chassis (large tower), is among the first generation of Power Macintosh computer and the utilizes the server market designation of 50 in the name.
Finally, the digits following the slash indicate the microprocessor clock speed.