2.11 Short And Long Commands

A short command is a command whose argument may not contain a paragraph break (either as a blank line or using \par.) Conversely, a long command is a command whose argument may contain a paragraph break.

Using short commands helps to test for forgotten braces, so it is recommended that when you define a new command you should always make the command a short command, unless there is a chance that the argument may need to contain a paragraph break.