Where are numbering schemes defined?

Numbering schemes are defined in table_num_scheme.
A simple SQL query will give you all of them:
select * from table_num_scheme

If you are using Dovetail Admin (fcAdmin), click on the "Numbering Schemes" menu item, and you will be able to see them all, as well as manage their properties.
You can also add new numbering schemes.

If you're using the Clarify Classic Client:
Apps - Policies and Customers
Setup - Numbering Scheme

• Dovetail Admin – Dovetail Admin (formerly fcAdmin)
• Dovetail SDK – Dovetail Software Development Kit for Clarify