Some topics exist in more than one section. To demonstrate this, let's use the whatis command, which shows all the available man pages for a topic:

$ whatis printf
printf               (1)  - format and print data
printf               (3)  - formatted output conversion

In this case, man printf would default to the page in section 1 ("User Programs"). If we were writing a C program, we might be more interested in the page from section 3 ("Library functions"). You can call up a man page from a certain section by specifying it on the command line, so to ask for printf(3), we would type:

$ man 3 printf