From Wikipedia:
On 1 July 1899, the Royal Artillery was divided into three groups: the Royal Horse Artillery of 21 batteries and the Royal Field Artillery of 95 batteries comprised one group, while the coastal defence, mountain, siege and heavy batteries were split off into another group named the Royal Garrison Artillery of 91 companies.[1] The third group continued to be titled simply Royal Artillery, and was responsible for ammunition storage and supply. Which branch a gunner belonged to was indicated by metal shoulder titles (R.A., R.F.A., R.H.A., or R.G.A.).