A stool was a type of seat.
In a police box, there was often a writing desk and stool, as well as an electric heater. (AUDIO: The Eternal Summer)
Altos was once hit over the head with a stool. (TV: The Keys of Marinus)
The Duke of Manhattan's assistant Frau Clovis threatened the Tenth Doctor and a Cassandra-possessed Rose Tyler with a stool, believing them to have been infected by the plague-carriers that were roaming through the New New York Hospital. (TV: New Earth)
When she and the Doctor were accosted by a group of Ood shortly after arriving on Sanctuary Base 6, Rose, assuming the creatures were hostile, grabbed a nearby stool to defend herself. (TV: The Impossible Planet)
The Seventh Doctor's question-mark umbrella handle was known to split in half and unfold into a seat, allowing the Doctor to plant the umbrella into the ground and use it as a makeshift stool. (COMIC: Planet of the Dead, PROSE: Storm Harvest)
Iris Wildthyme once had a bar stool on the Celestial Omnibus. (PROSE: Enter Wildthyme)