I tried fitting two in an Asp and then went scooping they act like an expansion of the internal tank that can fit in a generic cargo bay.
I think the functionality will be useful if you have a group of ships in hostile territory consisting of short range fighter class ships (e.g. Vultures) that need refuelling on a regular basis, having a T9 tanker would allow them to keep on station without resorting to scooping which could attract attention.