When a !include is the first string on a line in a wiki page, then the argument of the !include is presumed to be another wiki page. The contents of that wiki page are inserted in place of the !include.
The following is a test that demonstrates how this works.
First create a page to be included.
script
start
Page Builder
line
included
page
IncludedPage
Next create a page to do the including.
script
start
Page Builder
line
before
line
!include IncludedPage
line
after
page
IncludingPage
Then request the including page.
Response Requester.
uri
valid?
contents?
IncludingPage
true
Examine the requested page to be sure the included page was in fact included.
Response Examiner.
type
pattern
matches?
value
contents
included
true
Now look at every line on the including page to make sure that the inclusions were done in order.