CCA
"Just wanted to be clear as others had said that it’s not a tank that’s installed in the cargo areas."
Well yes and no.
The RCT does indeed occupy part of the space that in the non-XLR A321 is used for cargo or, optionally, for the removable ACT(s) in the A321LR.
The difference is that, while the ACTs are able to be removed, the RCT is permanently built in to the aircraft, and thanks to its shape the single RCT holds more fuel than 3 ACTs combined while sacrificing less cargo space.