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In a discussion associated with this answer a comment includes the following:
The ISS is the only spacecraft in the world that has experience with regular on-orbit refueling.
What is the extent of on-orbit refueling experience at the ISS? Which fuels? which spacecraft?
The propulsion system of the Service Module (public relations name: Zvezda), part of the Russian side of the ISS, is refueled by Russian Progress spacecraft, and formerly by European Automated/Ariane Transfer Vehicle (ATV) spacecraft.
Both Progress and ATV spacecraft deliver approximately 850 kg of propellant to the ISS.
The Service Module uses storable dinitrogen tetroxide and unsymmetrical dimethylhydrazine propellants.
The propellant is transferred through ports on the docking interface.
This schematic drawing shows the tankage in the midsection of the Progress.
In addition to the standard operational refueling missions that are part of normal station-keeping operations, there have also been experiments performed that studied in-space refueling.
In particular, three phases of the NASA Robotic Refueling Mission have been completed, which simulated on-orbit refueling of a satellite not designed for refueling. A module was installed on the outside of the ISS that mimicked the fueling connections on a typical satellite. Note that "fueling connections on a typical satellite" here means the kinds of connections, valves, seals, etc. that you would find on a satellite not designed for refueling, to be only used once, on the ground, for initial fueling of the satellite.
Using these connections that were never intended to be re-used and never intended to be used in-flight, the RRM successfully demonstrated on-orbit refueling of hypergolics, oxidizer, and cryogenics.
