It's a good question. Basically, it's a "high octane" tune. The changes between 91 and 93 octane are minimal, so in most cases, 91 or 93 octane can be used. If there are any driveability issues caused by the fuel, the timing can always be adjusted to compensate.
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~ Corey 
Resting in Peace - 
1969 - 2019
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