This is specified in the spec, for example here :
http://www.biblioscape.com/rtf15_spec.htm#Heading2
« The delimiter marks the end of an RTF control word, and can be one of the following : [...]
* A digit or a hyphen (-), which indicates that a numeric parameter follows. The subsequent digital sequence is then delimited by a space or any character other than a letter or a digit. »
