The register asks which one you're on as soon as it loads, and it won't let you past until you answer.
Each row shows the register's name, its number, terminal type and serial number. The one you used last is highlighted. Click yours.
Why it matters
Card payments go to the terminal attached to the register you pick. Choose the wrong one and the card prompt appears on somebody else's pinpad.
Your cash drawer session is tied to it. Opening a drawer, closing it, and everything in the Cash Drawer Report is per-register.
Cash, check, gift card, loyalty and vendor balance payments don't check for a register. They'll go through with none selected — but they're then recorded against no register at all, and won't show up when you filter reports or reconcile a drawer. Always pick your register, even on a cash-only shift.
Changing it
Press the register button in the right rail at any time. This time you can close the window without choosing.
Your choice is remembered per workstation — it's stored in that browser, not against your login. Switch machines and you'll be asked again; that's expected.
If your store uses a customer display
Picking a register also opens the customer-facing display, if your store has it enabled. That's deliberate — browsers only allow a second window to open in response to a button press, so this is the moment it can happen. See Using the Customer-Facing Display.
If no registers are listed
You'll see "No registers are configured for this store. Add them in Admin, then reload." Registers are set up under Admin → Payment Terminals.