Here https://wiki.debian.org/SourcesList sources.list format is explained:
The entries in this file normally follow this format:
The 'distribution' can be either the release code name / alias (jessie, stretch, buster, sid) or the release class (oldstable, stable, testing, unstable) respectively. If you mean to be tracking a release class then use the class name, if you want to track a Debian point release, use the code name. Avoid using stable in your sources.list as that results in nasty surprises and broken systems when the next release is made; upgrading to a new release should be a deliberate, careful action and editing a file once every two years is not a burden.
I am a developer and I can't understand what they mean by
the release class? How to explain it? For example, when we develop a project we have version 1, 2, 3. We can give any version own name - it is cool, isn't it. But what is the release class?