Although it sounds a lot like bureaucracy, it is the Debian Policy Manual that makes Debian the best operating system. It describes what packages should contain, how they should be configured, and generally how packages fit together to create a Debian system.
There are policies which govern every package. The most important one is that Debian needs to comply with the FilesystemHierarchyStandard - this makes sure you can always find files where you expect them. There are specific policies which govern which interfaces certain packages follow. For example, every program that sends e-mail needs to Provide: mail-transfer-agent. This means that other programs that need to send e-mail just depend: mail-transfer-agent, and are able to use /usr/sbin/sendmail.
Development
See the Debian Policy team description for information on how Policy is developed and changed.