This is a terrible idea, but not easy to make the users aware and avoid it !
From the user point of view
It is tempting, because in every report and on every screen, you can immediately recognize the type of customer, and you can easily restrict a report to a type of customer by playing with the number range.
This is so popular that THE leading ERP even allows to assign every customer type to a (potentially different) number range.
But in practice, a meaningful numbering convention comes with some disadvantages. For example:
- what if one day you have 30K+1 customers (it doesn't mean that much active customers, but it could be also old customers that you have to keep for 10 years due to legal requirements).
- what if one customer could exceptionally correspond to two types of customers (e.g. a retail customer that starts to resell some goods and become a B2B customer for some transactions but not all) ?
- what if a customer changes customer type (e.g. corporate group customer who is carved out after an M&A and becomes a third party customer; or an non-profit organization that cahnges statutes and becomes a private company; etc...) ?
From my own experiences, these situations are very often denied by the users ("it never occurs") but happen most of the time within the first year of the implementation (mostly the third case), leaving the people clueless about how to tackle it.
From the technical point of view
Well, you know the picture! In fact your code is designed to work with one counter and now you need to change to two counters (or change the query that gives you the new number). Not to mention the magical 30K number. Not to mention the hidden relation and the risk of inconsistencies.
How to convince the users to accept a neutral numbering ?
I can warn you after some 20 ERP implementations that it's not easy to convince users that it's a bad idea for a technical reason. So the best is to work on the risks from the user point of view.
Now another angle to start the discussion is to ask why they came out with this new numbering scheme. Try to challenge this requirement. In my experience, very often it occurs that either the users are not aware that customer type is already managed in another way, or the users face some practical issue (e.g. "this one report where we can't use the customer type as selection criteria") that brought them to consider this alternative. In this case you could use a what-if dialogue: "what-if in this report youc ouled easily select on the customer type ? Could you then work with a single numbering scheme ?").