13.3 I am making a special Linux distribution for a "vertical market". Can I use Debian GNU/Linux for the guts of a Linux system and add my own applications on top of it?

Yes. Debian-derived distributions are being created both in close cooperation with the Debian project itself and by external parties. One can use the Custom Debian Distributions framework to work together with Debian; Skolelinux is one such project.

One person is building a "Linux for Hams" distribution, with specialized programs for Radio Amateurs. He is starting with Debian as the "base system", and adding programs to control the transmitter, track satellites, etc. All of the programs he adds are packaged with the Debian packaging system so that his users will be able to upgrade easily when he releases subsequent CDs.

There are several other Debian-derived distributions already on the market, such as Progeny Debian, Linspire, Knoppix and Ubuntu, that are targeted at a different kind of audience than the original Debian GNU/Linux is, but use most of our components in their product.

Debian also provides a mechanism to allow developers and system administrators to install local versions of selected files in such a way that they will not be overwritten when other packages are upgraded. This is discussed further in the question on How do I override a file installed by a package, so that a different version can be used instead?, Section 10.8.