They do have independent battalions IIRC that replace a full infantry regiment in some RCTs too.
I'd imagine they get some separate missions away from the leg groups. But over all yes, its another part of the Suns combined arms support brigades for the RCTs.
Which is why I specified each RCT infantry regiment getting a company of BA the minimum goal. Also in cases of full BA Battalions they replace infantry regiments. I supose in cases of full BA battalions they likely replaced the jump infantry regiment.
From what I know the original mix was two leg infantry regiments, two mechanized/vehicle mounted reigments and one jump regiment. Around the 4th Succession War the FS started trying to shift the two leg regiments to mechanized, but never fully managed the transition. Probably the same deal with BA in Commonwealth, then later FedSun's service. A long term organizational goal, never quite achived due to events and budget.