I'm trying to describe a process where there is one stage that serves as a foundation (base) for the intermediate stages, and then a final stage that serves as, well, the opposite. The idea is that Stage A provides broad support that underpins the other stages, and then the final Stage X provides an overarching conclusion. I'm trying to draw parallels between the foundational stage and this one, so I'm trying to find set of words that describe the same type of function though at different ends of the process.
My sample sentence (using A and X as placeholder names) is as follows:
Stage X serves as the ______, opposite the Stage A foundation.
The closest word I could think of was endcap – for example, if you think of a fence-post, this is the cap that goes at the end. However the mental image I have of an endcap is a pointed triangular structure, which implies that all things converge, which is not accurate in this situation.