What does this icon mean?

When you open the inventory window, there is an icon for a backpack and then one that looks like a rock? or something. Sometimes items go into it, other times they don't. What does the icon represent and what can go/not go into it?

 

One of them is the backpack/cape/whatever slot. I've personally found a wizard's scroll and beer keg that go there, both stat-less. A friend found a cape that increased control-looking range.

The other slot is just the back's vanity slot, same as all the other armor slots.

 

It represents blocks that can be mined... i'm bad at explaining things so i'll try my best
it's really meant for blocks that you mine... like dirt, cobble, moonrock... so really all minerals thats really all i have seen go in it so far

also it's not restricted to JUST minerals as of now. You could put anything into it if you wanted to i'm pretty sure it's to help organize your inventory

 

Ah, that's what he was talking about...

Yeah, extra inventory tab that automatically tries to suck in all the useless dirt, stone and such.

 

It's pretty helpful if you ask me lol
For either extra inventory space or for when i'm just mining because if it were Terraria my inventory would of been filled in seconds

 

I personally wish that each bag could only be used for certain items. Because I hate when my build bag fills up and starts over flowing into my equip bag, and vice versa.

 

Ah ha, so it's meant for resources (dirt, rocks, etc), even though it's a free for all for now, I see. Thanks!

 

Yep, it looks like that tab is supposed to be used exclusively for building materials. Hopefully, they'll actually restrict it at some point.

Although, by then I'd like some inventory management tools.

Chests are so small and I have so much stuff. It's an inventory nightmare.