header api

have a specific question on headers and the related api.  according to another 
listmember (which I've forgotten), all headers should be used at most once. 
I've seen many apps that use the same header repeatedly to hold info specific to 
that app.  what is the preferred way of storing multiple lines of application 
specific info?  unique headers? multi-line header?  to what extent should the 
api support/enforce this info management ideal?