This would include a file on your site located virtually.
However since includes are inserted before the server processes the ASP you cannot use an asp variable to dynamically change the file being included.
You could have a series of (if/else) statements with different Includes within each one.
Example:
<% if bla = "1" then %>
<!--#include file="file1.asp"-->
<% elseif bla = "2" then %>
<!--#include file="file2.asp"-->
<% end if %>
You could also include another asp file and then have that file perform the tests neccessary to determine the page to be directed to.
Example:
<!--#include file="determinefile.asp"-->
And the contents of determinefile.asp would be.
<% if bla = "1" then %>
<% response.write("file1.asp") %>
<% elseif bla = "2" then %>
<% response.write("file2.asp") %>
<% end if %>