The ACT module handles NNID accounts.

ACT service "act:a"

ACT service "act:u"

Command Header	Available since system-version	Description
0x00010084		Initialize? cmd[1] = u32 ctrsdkversion_probably, cmd[2] = unknown, cmd[3] = 0x20(kernel PID header), cmd[4] = PID value written by kernel, cmd[5] = 0x0(handle-transfer header for kernel), cmd[6] = handle value.
0x000600C2		GetAccountDataBlock

This is the service used by regular applications.