" These workouts need to be performed precisely in order to be able to properly compare one result to the other". Does it mean that I have to start with the level 1, or I can start with for example 4 and proceed 5,6,7,8… ?
I think that the meaning is
When training keep the intensity as close as possible to the prescribed one, so you can compare several session of the same workout one against the other.
Did I get it right?
I do not see reason to repeat certain level. If I am able to finish it I will skip to the next level next time. Only when I’m not capable of finishing certain level I will repeat it in some time.
Armando, can you please comment?
As your level increases, you’ll need more motivation to increase levels and sometimes even just to repeat a level.
Jaroslaw — IMHO Hardness Level Tests are meant to test your level of Hardness, not your level of Fitness.
You can start at any level you think suitable to your current Fitness/Training/Hardness Level. From there you can shift up/down until you find the one that suits you best.
Once you’ve found it, my bet is that you will stick to it (+/-1 or maybe +/-2, depending what phase you are currently in: Prep, Base, Build, Peak, Off Season, etc.) for a long time to come.
Have a look here and you’ll find that “In general, levels 1-3 are for athletes recovering from injury. Levels 4-8 are generally levels trained athletes can expect to complete. Levels 9-13 are competitive levels. Level 14-16 are extreme and are the domain of professional-level competitors.”
To me that means that one cannot even think to go all the way to “Level 16”, but will sooner or later reach their limit which is dictated by their age/physiology/experience/training/limiters etc.
Though, as your fitness improves (MPA/TP/HIE/LTP raise) you might want to repeat the same test with the updated Fitness Signature values.
Then, that very same test will feel harder because of your new values. And here is where SMART Intervals algorithm plays the big role. It will automatically adjust the whole workout to take into account your new improved Fitness Signature.
The outcome of these Hardness Test are really meaningful only if performed properly. A poorly executed test will lead to skewed results.
This is even truer if you want to compare two session of the same test. (As you said: "Only when I’m not capable of finishing certain level I will repeat it in some time.")
Remember — In order to be able to properly compare one result to the other these workouts need to be performed precisely.
(Clauses switched on purpose. Possibly the “precisely in order” part is what you misunderstood 
).
Just my two cents.
Said perfectly.