Writeback - using dynamic tags

In later versions of the Writeback table extension it has become possible to use dynamic tags.
One such tag is {{APP.SUFFIX}}
When I try using this I get no result.

The Id and Name that are also possible gives me a result (I'm trying to use it as part of the subfolder).
My question is where {{APP.SUFFIX}} is supposed to take its value from?
Is it part of the app name, if yes then how does the extension where the normal part of the name ends and the suffix starts