You're right, this isn't the right term. (I was thinking that if the competitor's stack covers your whole business you open source an implementation, you don't

You're right, this isn't the right term. (I was thinking that if the competitor's stack covers your whole business you open source an implementation, you don't have a business. And so whatever business you have left after open sourcing (support, etc) is your new business, and the rest is complement. But this is getting pretty far from what the term normally means which is finding areas that are naturally complementary to your existing business and commoditizing them.)