[cypress66]

MANUAL SECTIONS The standard sections of the manual include:

    1      User Commands
    2      System Calls
    3      C Library Functions
    4      Devices and Special Files
    5      File Formats and Conventions
    6      Games et. al.
    7      Miscellanea
    8      System Administration tools and Daemons

[NobodyNada]

It’s worth mentioning that in online conversations or documentation, rather than referring to manpages specifically, it’s often used simply to disambiguate “the Unix command ping” from anything else that might be called ping. E.g. in a casual conversation the word “open” can refer to a hundred different things, but if someone writes “open(2)” it’s unambiguous that they’re talking about the syscall.

Kinda like how macOS users will write “Mail.app” to disambiguate the built-in email client from the concept of email in general.

[bxparks]

And it's used to disambiguate the same name in different sections, for example:

   $ man 1 printf # prints printf(1)
   $ man 3 printf # prints printf(3)
   $ man printf   # prints printf(1)

[Arnavion]

(See `man man` in your own installation.)

[ciupicri]

For OpenBSD that would be http://man.openbsd.org/man.1#s

[Tempest1981]

Further details here: https://unix.stackexchange.com/a/3587