Meaning of development version numeric suffix

Hi everyone,

When I look as an alpha, beta or RC version of WP, I see that it always contains a usually big number.

For instance, current trunk version of WP is 4.4-alpha-33643.

Now, in a compiled language (e.g. Java, C# or whatever) this number may represent the build number.

But in PHP you don’t actually build anything.

What is that number?
How is generated?

I must admit that I’m looking for something similar to use in my development, especially when publicly releasing a non final version, but I never found a decent way, except than manually increasing this number.

I suspect that one of the possible answer to my question is that the core team runs the code through some CI software/service and on every iteration that pass all checks and tests, you increase that number and add it (automatically? how?) to the version.php, but this is just a blind guess ??

Thanks.