In the four-level hierarchy that characterizes a MoStack the stack is the first, the topmost level.
MoStacks currently is a strict one-stack-at-a-time system: You can work with one stack only. If one is opened, and you want to see another, you have to close the current stack and open that other stack. There are no connections between stacks; each stack is a world for itself.
Because stacks can contain many different things, MoStacks implements a rather flexible file format to store them. You find details about this proprietary binary format in the chapter File Format. Be careful with your stacks, as the file format is too complex to salvage data in case a stack gets damaged. See also chapter Backup Files.
With this single-stack system there is a need to be able to transfer data between stacks, but unfortunately such functionality is still missing.