According to Wikipedia: The Royal Garrison Artillery (RGA) was an arm of the Royal Artillery that was originally tasked with manning the guns of the British Empire's forts and fortresses, including coastal artillery batteries, the heavy gun batteries attached to each infantry division, and the guns of the siege artillery. By comparison, the Royal Field Artillery (RFA) of the British Army provided artillery support for the British Army. It came into being when the Royal Artillery was divided on 1 July 1899, it was reamalgamated into the Royal Artillery in 1924. The Royal Field Artillery was the largest arm of the artillery. It was responsible for the medium calibre guns and howitzers deployed close to the front line and was reasonably mobile. It was organised into brigades, attached to divisions or higher formations.