Custom webapp is for installing things outside of the yunohost repo. ie a static html website.
If I wanted to install another html website I would need another custom webapp instance.
So in theory if I wanted to install Libretime to run a radio station I could install it in a custom webapp instance. Would I need to put php and mysql databases in that instance or would those remain outside of it?
I guess what Im saying is it a container with everything that instance needs goes inside it?