Easiest way to define ProductName variable?

I'm brand new to docbooks, having inherited it through this job I started. Now that I have my build scripts working, I'm in good shape, except that I've not yet figured out how best to achieve this simple goal: create a variable (such as ProductName) that can resolve to one of several values based on whether it's a corporate or an OEM build target.

I must not be searching on the correct words to find out how this is typically done!

Gratefully,

Mary