The RR-3 “Stolon” Tanker is an essential vehicle for Foxhole logistics as it's the few vehicles that can efficiently transport liquids from one point to another. While there are other structures and vehicles that can do this (and at a much higher capacity), it is far easier to move to locations and requires less infrastructure to do so.
Like all structures and vehicles that can refuel, it will keep track of how many liters was extracted from it (i.e. if there was 500L inside, and only 37L was extracted, it now has 463L stored inside). Although it has an internal counter, it is not visibly shown when looking in its inventory/stockpile.
Early-game Facility Logistics
Prior to Tier 2 Facilities unlocking (which includes pipes), Fuel Tankers are extremely useful for supplying liquids to facilities that need them, like the Diesel Power Plant. Using them also cuts down on unnecessary building, which increases msupp consumption. Depending on how long Tier 2 Facilities takes to unlock (which can take between 6-7 days to unlock), not building unnecessary things will reduce burnout of players maintaining them at the start of each war.
Prior to their rework in Update 1.50 ('Inferno'), Fuel Tankers were the only mobile structure/vehicle in the game that could transport fuel. This was later changed with the introduction of the Liquid Container, which resulted in Fuel Tankers being nerfed.
It had 2 inventory slots, which could stack up to 100 per slot (giving it an effective carrying capacity of 20,000L).