Maybe an example will help it make sense.

The Standard Mileage Rate includes some built-in depreciation.  The amount changes for each year, but for 2024 that built-in depreciation is 30 cents per mile.

Let's say a taxpayer buys a car for $50,000 and drives 100,000 miles in 2024.  The built-in depreciation is $30,000.  That means the Adjusted Basis is $20,000 and that amount is used to determine if it is sold at a gain or a loss.

Now let's say the car only cost $20,000 and drives 100,000 miles in 2024.  The built-in depreciation is $30,000.  However, the Basis can't go below zero so the Adjusted Basis is $0 and that is what is used to determine if it is sold at a gain or a loss (you essentially got $10,000 of depreciation for free).  It is saying you can continue to use the Standard Mileage Rate even though the built-in depreciation has already lowered the Adjusted Basis to $0.

Does that help at all?