What is an RFC attachment, and why?

[Samuel W. Heywood]

Hello:

Recently I have received some emails with attachments identified
as files having an "RFC" extension.  Clicking on the attachment
ikon does not work.  Absolutely nothing happens.  When viewing source 
I find these attachments as appearing to be only ascii text files. 

What is an RFC file, and why do the senders of such attachments 
identify them as such instead of naming them simply as TXT files? 

Regards,

Sam Heywood