What Suffixes to use and when?

Hi:

This may be a very ignorant question (what questions aren't), but is 
there a reference of some sort that explains what suffixes should be 
used to call certain operations or languages?

For example, if I was to try to execute a XSSI command, the calling 
file must have the suffix of ".shtml". No problem there, I understand 
that.

However, while this may be obvious to everyone here, but I just tried 
to execute a --

<!--#include virtual="my.txt" -->

-- from within a ".shtml" file and the XSSI command in the text file 
(my.txt) didn't work. However, when I simply changed the suffix of 
the called text file from "txt" to ".shtml", such as --

<!--#include virtual="my.shtml" -->

-- everything worked as I wanted. This was very confusing to me 
--from my perspective, who would have thunk it?

I can't help but think that when using html (or shtml) to call xlm , 
xsl, css, and other such suffixes, there should be a list/register 
that says "This is OK to call this operation using this suffix, but 
you can't do that operation with that suffix." -- if that makes any 
sense.

Many thanks for any enlightenment.

/tedd


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