Questions about :command and how the user can define custom commands.

Vim allows users to define their own commands with the following format:

:com[mand][!] [{attr}...] {cmd} {rep}

The {cmd} part has to begin with a capital letter to differentiate built-in commands from user-defined commands.

The {attr} can be used to define a number of arguments taken by the command or the source to use to autocomplete these arguments.

Some related commands are :delc[ommand] and :comc[lear] which respectively allows to delete a user defined command and to clear all the user defined commands.

For more informations see: