I think the phrase you are looking for is "deep doctrine". In the LDS church it is common to refer to things that are taught often, well explained and well understood by the average member as the "doctrine" of the church. On these matters the "church" has an official stance or "declared doctrine". Examples of this would be things like faith, repentance, baptism, ordinances, priesthood etc.
Things which the church has no official position on, mainly because the have little bearing on "salvation", are not considered doctrinal and in many instances are speculative.
People (like me) who delve into these areas are considered to be exploring "deep doctrine". Some of what people call deep doctrine is just lesser known doctrine(because it requires in-depth study to understand) and some is not doctrine at all (pure speculation).
Right you are, thanks for the correction.