The goal of this software is to address the gap in the open source ecosystem. This multidimensional approach for modeling (and financial planning/forecasting) is over 40 years old. Systems like TM1 by IBM, Anaplan (to name just a few), have been using it for many decades.
The reason for using the word 'cube' is mainly familiarity of users with the concept of OLAP cube (https://en.wikipedia.org/wiki/OLAP_cube), which is what this tech should perhaps be called.
As for data input... indeed, there is some room for improvement: there are 3 ways of editing a rule: via the rule bar, directly on the grid, and modifying an existing rule using the rule panel. We need feedback to decide which way to keep and what to remove. UX/UI is extremely tricky. Oh, there's also a way to insert a rule via the terminal (text user interface command line).
As for operation on slices, a slice can be defined using specific items in a dimension and omitting others (which will create a data 'slice'): for example if you have a 'cube' D with dimensions A B and C, and each dimension has items 1 2 and 3, then you can define a sum over a slice through this cube via this rule "=sum(D::a.1)" or, more explicitly, "=sum(D::a.1:b.:c.)", if written directly in a cell.
You can fill the whole cube with some value, say, 1, by defining a rule `* = 1` in that particular cube via the rule bar. The rule should produce the total of 9.
I hope this is helpful. Feel free to join the project's discord server (https://discord.gg/GfU5ypAbaD) I will be happy to explain.
PS
The functions are documented here: https://cloudcell.github.io/om-docs/reference/functions/
There's an Excel import plugin (available via menu 'Plugins') that allowed importing basic Excel files (stripping all formatting, however), but I guess it might need a few hours of work at the moment.