Dynamic language documentation

I see in a lot of documentation something like the following ...

fs.openSync(path, flags, [mode])
Synchronous open(2).

What is meant when someone references a method with a number in the parenthesis? Is this just referring to the function signature with 2 parameters in a dynamic language. What is the point of this?

It refers to the Unix man pages, and the number 2 refers to the section of the man pages.

At a Unix command prompt type: man open

Or to specify it more completely: man -S 2 open

The 2 section refers to Unix kernel functions, while the 3 section refers to C library functions.