Unknown Aquarium Equipment

[DirtyPanda]

Hi there Aquarium Advice!
I recently bought a 55 off craigslist and the tank came with a very strange feature...

A part of the tank has been sectioned off and within that section are a coarse sponge, bioballs, and pump. Is this a filter? Refugium? Wet-dry?
I will dry to add pictures, thank you!

 

[Fishfur]

Probably used as a refugium or sump, possibly an attempt at a Hamburg Mattenfilter, maybe.