
In Auto Racing, the Yellow Flag means that there is a caution out. In NASCAR, its generally just a caution while IndyCar and FIA-sanctioned championships have a few more rules in place.
In IndyCar, the yellow flag means Local caution (single wave), Full-Course Caution (twin wave), or just a regular caution.
While in the FIA-sanctioned championships, the yellow flag means a Local Caution, Full-Course Caution (with letter board), or a Slow zone on track (with SLOW board). The Slow zone limits the cars to 80 kph (50 mph) and speeding in the slow zone results in a 60 second stop and go penalty where nobody is allowed to touch the car resulting in an isolated pit stop.
In iRacing, the yellow flag is just there to mean a caution in general with a Local Caution (single wave) and has no double wave feature, slow zone feature, or any letter board or SLOW board feature.
NASCAR: Caution.
IndyCar: Caution, Local Caution (single wave), Full-Course Caution (twin wave).
FIA-sanctioned championships: Local Caution, Full-Course Caution (with letter board), Slow zone on track (with SLOW board).
iRacing: Caution, Local Caution (single wave).