When I try to open an *.xml document that uses the 40051-2C XML architecture, it opens with no problems.
When I try to open a *.dita document that uses the DITA XML architecture, it tries to open Arbortext with Styler and I get a message about no license. If Arbortext is already open, the *.dita document opens with no problems.
I've looked through Windows file associations, environment variables, and the C:\flexlm directory, but have had no luck. The documents use different DOCTYPE declarations (DITA versus USA-DOD) but I can't change that.
I tried it also and it went to open Arbortext Styler. However, since we use Windchill to manage our DITA and DOD 40051 documents and objects we start with Arbortext open and go through the AT interface to access the objects stored in Windchill, hence no one has expressed any issue. I will do a bit more investigating, seems to be a strange occurrence.