NX Open is actually a term which refers to NX programming tools in general, including what was once called User Function but now falls into separate areas based the programming environment used, such as C/C++ versus JAVA versus .NET, etc. Also included in this 'grouping' is GRIP and sometines people also include KF and I suspect will soon be including SNAP.
Below link may helpful for you..
CAD Automation Engineer
NX 9 | NX 11 | Teamcenter 10