I've never heard it called a poop door. It's always the pop hole door or pop door. Maybe you read a typo? Or is that a regional term where you live?
Sometimes people do add extra little doors to clean under the roosts or to gather eggs. I usually refer to these as service doors. They're especially handy on small elevated coops that are too short to walk into. Usually those are for a few chickens on a smaller lot. For larger numbers of chickens, the most common design is usually more like a shed, with modifications.
Have you decided on a coop style, yet? There's a link to coop designs at the top of the page, in case you haven't found it, yet. I love how different they all are, when people make their own designs.