If lunch meat says it’s 97% fat free, that means it’s 3% fat. Fat is 3% of package weight.
Sanka says it’s 97% caffeine free.
So is it 3% caffeine? = 1/2 oz caffeine per 1 lb package
Or 3% of the normal amount of caffeine?
Or something else?
Posting at Fathom
3% of normal caffeine level, is what I think they mean.
But it’s an interesting question, taken to a more general level – what do advertisers mean by those comparative percentages?
It’s like 2% milk – ever wonder what the other 98% is? 
In the case of milk, what they’re saying is that it only has 4/7 of the normal amount of cream (“part-skim” is the old way of phrasing that) – normal homogenized milk has approximately 3.5% cream content, depending on the predominant breed of cow milked, etc. In 2% milk, that is reduced from 3.5% to 2% before homogenization, and similarly for 1% and 0.5% milk.
But there are other foods where the Sanka-style percentage is done: the foodstuff may normally be 0.8154% a given substance (“X-eine”) prior to processing, and that’s reduced to 0.2037% and the claim is made that it’s “only 25% the normal X-eine content.” True – but how pertinent to health matters?
I wouldn’t be at all surprised to find out that’s why they market it that way!
Sort of like a mother-in-law.
What is supercritical carbon dioxide?
Warm and lotsa pressure, just like a mother-in-law.
Caustic and gassy, like my ex. 
So Polycarp tell me all about the **.**56 % impurities contained in Ivory soap.