From the guide:
General Support and Direct Support off-map artillery
General Support and Direct Support off-map artillery types have been added for off-map artillery purchase.
Direct Support is less responsive than normal arty, General Support even less so. The delay is longer for calls for fire, other than onto gold spots or as a pre game bombardment (their main use). Shifting fires costs a little more for these type of batteries and they are less likely to be in radio contact as they are theoretically shared with other formations than yours. As well, they are less likely to fire counter battery fires if left idle. Strike air types may be bought in general or direct support mode if desired.
Direct Support costs 75% of a full Under Command battery and General Support costs 50% of a full Under Command battery.
A button on the purchase menu cycles between artillery types. If General Support or Direct Support purchase is active, then only the appropriate formations are filtered to show on the arty page and the miscellaneous page (for strike air). Cycle this back to Under Command artillery to see the filtered-out formations again. General Support and Direct Support off-map artillery cannot be purchased for your core in a campaign
If there is an inbalance then it is in the players favour as the AI tends not to shift fire but replots.