Can someone explain this diagram about Slab Allocation?

Question

I'm trying to understand how Slab Allocation works and why it is different or better than ordinary paging.

I found this diagram which I believe would be helpful if it had more explanation.

Some questions:

• What do the 3KB and 7KB items represent? Must they be related somehow? Why are they packaged that way?
• In the caches column, are the caches the grey boxes, or the white/blue boxes inside the grey boxes? Are the grey boxes a package of caches?
• Are the slabs just the blue boxes or is the whole "Physical Contiguous Pages" a slab?

I'd really appreciate some help. THanks!