It's just to determine the concentration of oil in the product. The more there are, the longest lasting it will be.
Eau fraiche is basically your aftershave. Last 30 minutes to an hour.
Cologne tends to last about 2 hrs or so. 5% or less.
Eau de toilette (EDT) usually last a bit longer but not by much, around 3-4 hrs. Typically around 5% to 10%, some go to 15%.
Eau de parfum (EDP) has a higher concentration and can last anywhere between 5 to 8 hours. Around 20% oil concentration.
Parfum, is by itself at the top if we exclude pure oil. Easily lasting a day. Can go to around 30% and even 40% in oil concentration.
Obviously the higher concentrations are more expensive. It also entirely depends on your use case and how long you want/need to smell.
I don't know if there are actual guidelines in the industry for these. This is just info I found online.
But please. Do everyone a favor... Don't empty the entire bottle on yourself. 2 sprays at the minimum. 5 MAX.
I swear some people are spraying this stuff all over themselves and you can still smell them 10 minutes after they passed through an area.