Rithmic systems and gateways, explained
When a Rithmic connection will not pass Test, the wrong system is the usual suspect. Here is what a system actually is, how to find yours, and why MirrorFill never asks you for a gateway.
What a "system" is
A system is the Rithmic environment your firm or broker runs. Prop firms operate white-labeled Rithmic systems under their own names (Apex, TopstepTrader, Leeloo Trading), and Rithmic runs a few of its own, like Rithmic Test, Rithmic Paper Trading, Rithmic 01 and Rithmic 04 Colo. Your credentials exist on exactly one system. Pick a different one and the login fails, just like a wrong password would.
Rithmic's own systems
Rithmic's own production systems (Rithmic 01, Rithmic 04 Colo and Rithmic Paper Trading) are in the dropdown and selectable like any other: Rithmic reviews every application before it may reach a production system, and MirrorFill has passed that review. One thing to know before picking them: Rithmic's production interface does not work with demo trials, so the login needs a paid market data subscription behind it.
Prop-firm systems are unaffected by any of that. If your accounts are with a firm, your system is the firm's own entry, exactly as before.
How to find yours
Your firm's welcome e-mail names it, and in the dropdown it is usually just the firm's own name. When the Add connection form opens, MirrorFill asks Rithmic for the live system list and fills the dropdown from it, so in most cases you scroll and pick. The dropdown starts on "Choose your system", so nothing is picked for you by accident.
If the live lookup fails (you are offline, or something on the network blocks it), the form says "Showing the standard list" and falls back to known systems: Rithmic Test, TopstepTrader, Apex, LucidTrading, MES Capital, TradeFundrr, UProfitTrader, Leeloo Trading, Rithmic 01, Rithmic 04 Colo, and Rithmic Paper Trading. Your system is very likely in that list too.
One wrinkle is handled for you: Rithmic serves its Test system from a different connect point than the production systems. Picking a system in the dropdown points the connection at the right one automatically.
Why there is no gateway picker
A gateway is only the network route to a system. It is never part of your login (Rithmic does not receive it as part of who you are), so choosing one cannot make a login pass or fail. Since there is nothing to decide, MirrorFill fixes every connection to the Chicago Area route and does not ask. One less field, one less thing to get wrong.
Quick answers
Is the system the same as my broker?
Not exactly. The system is the Rithmic environment your accounts live on; your broker or firm chooses which one it runs. In practice, the firm's name in the dropdown is the answer.
What happens if I pick the wrong system?
The login fails, because your credentials do not exist on that system. Pick the right one and press Test again. Nothing else needs to change.
Can I select Rithmic 01 or Rithmic Paper Trading?
Yes. They are Rithmic's own production systems, and since MirrorFill passed Rithmic's review they are selectable like any other entry. The login needs a paid market data subscription behind it, because Rithmic's production interface does not accept demo trials.
Can I change the gateway?
No. Every connection uses the Chicago Area route, and because the gateway is never part of the login, changing it would not fix a failing connection anyway.