Let me see if I can be clearer: the definitions seem artificial and illogical. Something you use to label a formation but without any clear reason why you are doing so.
The reason couldn't be more clear or logical.
Let's pretend for a moment that we occasionally saw a strange phenomenon in the way light dispersed into a rainbow (pardon if I'm not perfect in my term usage here). For some strange reason we see a rainbow with a few missing colors (say orange, yellow and green).
If we applied the geological term "unconformity" to this situation, it would be the contact point between the Red and the Blue. In other words, it's the point where two things meet that would not normally ever meet each other.
And someone please correct me if I'm wrong - I'm new.
But it is only theory that says there is a layer missing from the strata, it's all a mental construct invented to accommodate the Old Earth.
And that's where you'd be wrong.
Just as the colors of the rainbow are established and identifiable, so are the layers of the earth. Methods of identification are numerous, broad, independent and consistent. The original methods were simple enough that they were developed and done near two centuries ago (by YECs). In the time since, the methods have become more and more detailed, precise and most importantly convergent.
Perhaps a course on how in any field, converging lines of evidence lead to the truth.