The magic phrase used on the site you posted is "California source income", (usually written as "California-sourced income"), which they conveniently don't define on that page.
Here's a non-authoritative site
https://www.palmspringstaxandtrustlawyers.com/nonresidents-working-remotely-for-california-businesse...
that does though have a plain-English explanation of what that phrase means (emphasis added:(
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California Tax Rules For Remote Workers
Generally if you work in California, whether you’re a resident or not, you have to pay income taxes on the wages you earn for those services. That’s due to the “source rule”: California taxes all income with a source in California. And for tax purposes, the source of income from services is the location where the services are performed. This is true even if you are a nonresident, even if the contract with the employer is made out-of-state, and even if the wages are paid outside of California.
You can imagine how important the source rule is for California’s taxing authority, the Franchise Tax Board, when it comes to actors and athletes. When LeBron James travels to California to play the Clips at Staples Center, California gets a cut of his pay for that night.
But what if the employee is a nonresident who doesn’t have to set foot in California to perform his services? Then the source rule works for the nonresident. Remember, the source of the services is the location where the work is actually performed. A nonresident programmer who monitors and upgrades satellite dish software for a Los Angeles-based media company, all while sitting comfortably in front of his computer in his Austin, Texas condo, doesn’t earn California-source income and doesn’t have to pay California income taxes.
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