Mime Multipart Nexpart ID description/naming

Hi,

I need to find out what the format of the ID used by clients e.g.

Content-Type: multipart/mixed;
	boundary="----=_NextPart_000_0000_01CA8430.8B71D430"
What is the construction of the string 000_0000_01CA8430.8B71D430  ?
does it indicate a pc name/datetime or client type?
Thanks