When someone comes back
If your staff banned a player and RealAC thinks that player is back on a new account, it tells you:
It shows up in game for staff with alerts on, and on your Discord webhook if you have one configured.
What it means
It means the way this account plays closely resembles a player you banned. Not that they share an IP, not that they share a machine. Behaviour.
It takes a few minutes of real play on the new account before there is anything to compare, so expect the alert while they are still online rather than the moment they log in.
What it is not
It is not a verdict, and RealAC will not ban anyone for it. Two people in one house share a lot of habits, and so do two friends who learned to play together. The confidence level tells you how strong the resemblance is. The decision is yours.
Use /realac alts to see what is currently outstanding, or /realac alts <player> for one account.
What to do with one
Treat it as a name to look at, not as evidence. Spectate them. Check whether the account trips anything on its own. If it does, you now have both.