The design of MoStacks is an attempt to make the best out of a rather difficult situation:
On the one hand you have a PC which is very powerful and fast, with more or less unlimited memory, keyboards and mouse for comfortable text entry and interaction, and large screens. But portability is of course a big issue, even if you have a notebook.
As a big plus, your smartphone is really portable and can run a long time on battery, but the screen is rather small, the keyboard even smaller, memory constrained and speed probably only just ok.
MoStacks tries to treat your PC and your smartphone together as a system and use both parts according to their strong points.
You use your PC for designing stacks, because thanks to keyboard, mouse and large screen it is much better suited for doing so than the smartphone. Look e.g. at the size of the window for defining fields and controls, note how many different things you can specify for each field, and imagine how hard it would be to implement this on the smartphone.
As explained in chapter Screen Size MoStacks on Windows tries to show you how your cards will look when displayed on the phone, but the visual correspondence is not perfect.
You also use your PC for large data imports, e.g. importing a CSV file. Such an import might take a long time on the phone, especially if you do it repeatedly until everything is right, and the result is easier to check on the big screen.
MoStacks on the smartphone is of course a good viewer for stacks, but also has a complete set of functions for data entry. You can create and delete cards and modify fields. The functions for configuring stacks are rather limited; you can't create new backgrounds directly on the phone or change the definitions of fields in existing backgrounds.
You can however change certain things that are difficult to get right on the PC. E.g. you are free to change the font of all elements directly on the phone, where you can see which fonts are really present and how a given point size plays out on the small screen.
The system as a whole becomes complete with a set of functions to easily transfer stacks back and forth between your PC and the smartphone. Backing up data entered on the phone on your PC is straightforward. You find the details about this in the chapter Transferring Stacks.