If your referring to a 500 I wouldn't know as never seen this 'diagram', I use common sense and that is based first and foremost on my safety.
Probably just as well as it's not particularly helpful or useful.
Like yourself, when I first got my car I used common sense to work out which hardpoints on the chassis I could use to lift the car safely. The handbook was of no use whatsoever & eLearn wasn't much better. I posted my findings
here & AFAIK noone who has lifted the car in this way has ever damaged either themselves of the car.
The problem comes when folks just lift the car using a trolley on the rail in the approximate position of the arrows. The rails bend & deform & the damage is cumulative. If you use two trolleys & lift it on both sides at once this way (foolish but it gets done every day at fast fit centres up & down the country), the risks (both to the operator and the car) just multiply.
Someone working with the care & diligence that T obviously shows doesn't need a diagram - they will use their knowledge, experience & sense of respect for customer's property to look for a safe place on the chassis to lift the car.
For the rest, I'd agree the markings on the car are confusing. The arrows on the sills are only there to show end users where to place the factory-supplied abomination of a scissor jack to lift the car for emergency wheel changing only - they don't indicate safe points to lift the car on the rail. It's been posted here that some tyre fitters have used the arrows as an excuse after they've collapsed the sills using a trolley jack.
If folks are inclined to get down & photograph their rails, we could collect some evidence as to the scale of the problem.
This issue isn't just confined to 500's (the Panda has exactly the same problem) or even FIATs - I've seen shedloads of cars of all marques damaged in this way. IMO manufacturers should place clear markings on the car showing where it can be safely lifted with a trolley jack. The alternative is that a succession of fast fit fitters will place clear markings on the car showing where it can't.