i can still remember when i didn't understand this!
to explain the 1/9th, 2/9th, 3/9th a bit further ( and i use the 9th as an example it was the same for the 5th to the 10th battalions)
the territorial force was created for home defence, at the start of the war, they were asked if they would served overseas, most said yes and wore their imperial service badge proudly. they formed the 1/9th, the remainder formed the 2/9th they trained and supplied the 1/9th with replacements and after conscription were sent abroad in their own right, the 3rd/9th was then formed to train and supply replacements for the 1/9th and 2/9th
as for A B C D companys (often shortened to Coys) a battalion was split into four companys of roughly 200 men
ok?