From the Oldbloods:
"The Grand Army of the Sovereign, also known simply as the Grand Army or colloquially as the Oldbloods, is imbued with many millennia of tradition, pomp, and grandeur. It has a standing force of several million troops, the majority infantry but including other types from shock cavalry to heavy artillery. Its members are possessed of a fearsome esprit de corps, and many companies are tithed en masse to the world’s Astra Militarum regiments such as the Juno Grand Fusiliers , where they find glory across the sector and beyond."