Grand battery is multiple artillery units that are adjacent combining their ranged fire dice into a single attack. It is ranged fire, and all the artillery units must be adjacent and have LOS.
Combined arms is a single infantry or cavalry unit attacking in close combat, and any ordered artillery with LOS to target can add their dice to that close combat attack. The artillery need not be adjacent to each other or the attacker, they simply need to be ordered and have LOS. The attack is a close combat attack using the sum of the attack dice.
So you could have 2+ artillery that could make a grand battery for a ranged attack instead be used for combined arms. You can also have 2+ artillery separated apart used for combined arms.
I think your confusion lies in the association of the artillery units as "a grand battery". They are not, they are simply a row of artillery. Grand battery is the attack they could make, but equally they can also make a combined arms attack. These are actions, not units.
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