What does Restricted Performance mean? When the check engine light is on?

I have a 2004 XJ8 and I found it to be bad gas. My vehicle likes Shell premium only 93 or 94. No Wawa, Exxon, Valero. You may need to put some gas treatment in, also run the gas out. I typically only need to pull over, before you actually need to stop your vehicle because mine will put,put after that. Turn car off, wait a few seconds and turn car back on, warning goes away and the gauge comes back to normal.