Army Orders was a communication mechanism used by the Army to its officers and men. Issued as required, Army Orders (AOs) covered a wide range of topics; officer passing examinations, exchange rates, changes in Army regulations and the award of some medals (DCM, LS&GC and MSM). the Orders were numbered sequentially by year.
There are bound copies of the AOs in the National Archives:
WO 123/41 Army Orders (War Office) 1899
WO 123/42 Army Orders (War Office) 1900
WO 123/43 Army Orders (War Office) 1901
WO 123/44 Army Orders (War Office) 1902
WO 123/45 Army Orders (War Office) 1903
The 1899 volume does contain many pages devoted to the mobilisation of the Army from October 1899 ie who was eligible for mobilisation, setting out different classes of mobilised troops etc.
There was one interesting item on changes to the khaki uniform.