255A concentrates more on setting up and maintaining the BCCS stacks. 255N covers some of it but focuses more on the network. 255A also covers more of the helpdesk functions depending on the unit. 255A's primary responsibility though is BCCS stack and its operation within Mission Command.
As a 255A that was a 25U, the bottom line is this. 255A deals with communication equipment that is used by a customer. You're responsible for integrating tech that goes to the end user. Where as the 255N is responsible for the network that said equipment operates on. Very little customer interaction as a 255N.
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