Well the side inventory was initially intended to be space for cycles. However doing this seemed like it would confuse newbies early in the game, let alone be a wasted space for a big part of the game. But the real reason we are scrapping the idea is because we have a much, much better idea. Sadly this idea will require quite some effort so it won't make it to the first public release. There are many more features that will come on later versions of sigma so you guys should definetly look forward to the project as a whole because it's gonna be good.
The only difference in inventory is that it's going to be bigger, it's currently 10x8 and will become 15x10 (double the size), compared to ultimative. It's 15x14 in the screenshot, but we are getting rid of some extra space to reserve it for this cool future idea.