I't likely Cisco has some logic to series assignment, but haven't read about it.

What I have noticed is Cisco usually seems to rank the different models into tiers of power based on the series number. Then increment series values for newer models in the same market tier. They also increment within the model of the series.

For instance market tiers of different ages:

Low-level tier - 2500 series

Mid-level tier - 4000 series

High-level tier - 7000 series

Low-level tier - 2600 series

Mid-level tier - 3600 series

High-level tier - 7500 series

Low-level tier - 2800 series

Mid-level tier - 3800 series

High-level tier - 7600 series

First number is a RANGE or TIER. Second number is a GENERATION from that RANGE, third and fourth is capabilities and performance of that particular Generation and Range.

RANGE E,g:

0 - up to 15 users

1 - up to 50 users

2 - up to 150 users

GENERATION:

8 - newest

7 - previous

6 - older

Capabilities and Performance:

01 - basic

20 - higher

50 - high

So if you interconnect RANGE of up to 150 users, of latest GENERATION, and high performance, you will get 2 8 50, which is 2850.

Hope this helps