I don't get this ; I often see something like "Power ratio : 3,6" or even "4" ?
You have read the marketing material

In reality, Specialized has published exact data about all full power motors here:
Specialized Full Power Motors
For instance, the 2.0E motor provides max assistance of 280% and max motor power of 430 W while the most expensive 2.2 motor gives a 410% max assistance at max power of 560 W.
You pay big money for the stronger motor, and the stronger motor is less economical as it draws more power from the battery.
It is similar if you bought one VW car with 80 HP and another with 150 (just an example). Of course, you pay more for the stronger car and use more petrol.
So, the "Eco mode" is about 1x assistance.
It depends what motor and what Assist % you can set in the Specialized App under the Tune. For instance 35% ECO for Como 3.0 motor would be 0.35 *2.8 = 0.98 or about 1x assist. Correct.
Use of Turbo depletes the battery very fast. That leads to frequent recharges, and each recharge is a step towards death in the battery lifetime. Given the UK's assistance speed limit of 15.5 mph, you would need Turbo on very steep climbs only, as Turbo used on the flat means almost immediate hitting 15.5 mph and maintaining that speed near to the cut-off. A bad way to use the e-bike, as the system cannot decide if you want to ride assisted below 15.5 mph or exceed that speed, and ride on leg power only with the motor not working. Setting ECO or SPORT keeps you below 15.5 mph with a solid motor assistance.
How do you know that exactly ?
As Jeremy tried to explain, there are two parameters that can be set in the Specialized App under Tune:
- Assist, for example 50% (0.50) is to be multiplied by max motor Assist (2.8x in case of the 2.0E motor). It gives the assistance of 0.5 * 2.8 = 1.4x. If your legs provide 100 W, the motor will help you with 140 W of assistance.
- Max Motor Power limit. If this second number is 50% (0.5) and the peak 2.0E motor power is 430 W, then the motor power is capped at 0.5 * 430 = 215 W. Whatever you do with your legs on the pedals, the motor won't provide more that 215 W. This parameter is meant to help saving battery power or making the ride more economical.