How to explain debian release code name and the release class?

Question

Here https://wiki.debian.org/SourcesList sources.list format is explained:

The entries in this file normally follow this format:

deb http://site.example.com/debian distribution component1 component2 component3
deb-src http://site.example.com/debian distribution component1 component2 component3

...

Distribution

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?

Answer 1

What is the release class?

There are multiple "versions" of Debian available during each of its release cycles. Cribbing from this official Debian Release overview:

Debian always has at least three releases in active maintenance: stable, testing and unstable.

stable

The stable distribution contains the latest officially released distribution of Debian. This is the production release of Debian, the one which we primarily recommend using.

[...]

testing

The testing distribution contains packages that haven't been accepted into a stable release yet, but they are in the queue for that. The main advantage of using this distribution is that it has more recent versions of software.

[...]

unstable

The unstable distribution is where active development of Debian occurs. Generally, this distribution is run by developers and those who like to live on the edge.

From the above, it can be concluded that oldstable likely refers to the prior stable release of Debian.