This is a sibling. 

When you open this via "sibling-a.txt" and save it you will see that the content of "sibling-b.txt" has also been changed. 
When you delete "sibling-a.txt" "sibling-b.txt" will still remain as one original file. 
When you delete "sibling-b.txt" "sibling-a.txt" will still remain as one original file. 

Sometimes this functionality is needed to e.g. share one PDF document under different url or translate a whole web page 
for multi-lingual support. Siblings also may have different properties. So it is possible to display the same content 
with different templates (which are attached to the template property). 