Wikipedia
A Field Ambulance (FA) is the name used by the British Army and the armies of other Commonwealth nations to describe a mobile medical unit that treats wounded soldiers very close to the combat zone. In the British military medical system that developed during the First World War, the FAs formed an intermediate level in the casualty evacuation chain that stretched from the Regimental Aid Posts near the front line and the Casualty Clearing Stations located outside the range of the enemy's artillery. FAs were often assigned to the brigades of a division.
During physical training such as "TAB'S" or booted runs, a field ambulance or "Jack Wagon" as it is known in this scenario will follow large groups of soldiers to pick up those who are injured or frankly cannot keep up, often it is a weakness of the mind rather than the body that results in soldiers taking a seat in the field ambulance/jack wagon. There is plaque on a seat in 7th Parachute Regiment Royal Horse Artillery jack wagon that says "Rob Fatback Phillips' seat" on account of the frequency he was sitting in it.
The term is no longer used in the British Royal Army Medical Corps. They were replaced by medical regiments (which are assigned to brigades) and field hospitals.