Back in the Citrix Presentation Server days, Citrix bought a technology called EdgeSight that could monitor user sessions, especially the ICA protocol which is patented by Citrix. EdgeSight was useful, but difficult to implement and use. Hence, a robust ecosystem of partners did a much better job of it.
Any partner could integrate with Citrix by just asking for the ICA specification and signing an NDA. Citrix would hand out the ICA protocol details and APIs to as many partners as they could. This was actually a good thing, because Citrix doesn't do reporting and monitoring very well. Later on there was some angst in business development circles about handing out the ICA protocol so freely, as some competitors ended up with the specification. It didn't really matter because this strategy has served Citrix well.
Citrix later added other features to the protocol and branded them as the High Definition eXperience (HDX), but it is still the same ICA protocol...