*** Solved, thanks again! The pipe is a drain from the patio, and the inlet is called an equalization port to allow drawing water even if the skimmer is full of leaves.
I finally got around to a partial drain / refill to correct CYA issues from previous owner. While in the process of all this, I've noticed two items in my pool setup that I'm not sure what the function is.
First, there is a pipe that I have no idea what it's for. It leads directly out into the alley behind my house, but I don't know where it comes from. There does not seem to be any valves to return water into this pipe, and there is already a different pipe that acts as overflow for the pool water. I had assumed I would be able to use this second pipe to drain my pool, but I couldn't figure out how to use it so I ended up siphoning water and using the pressure release valve on top of the filter to drain water.
Second, there is an inlet directly under the deep end skimmer in the pool. It seems to be connected to this skimmer, when I am pulling water from the skimmer this inlet has suction on it. I would have assumed it was for some kind of cleaner, but I already have a different port for the Kreepy Krauly. Is this just a second port for something else if I wanted it? Would it be wise to use it for something?
Thanks everyone! I'm well on my way to having my own trouble free pool thanks to you guys.