How can we determine the value of the player's impact on the style of play and what elements affect this influence negatively or positively?

When you choose a style of play, the information is displayed below, along with the impact of each player. You can see which characteristics requiring 'overtraining' are to be favoured for each style.
The positive impact you have in your chosen style is therefore negative for your opponent, and vice versa.

wildzap: Quand tu choisis un style de jeu, les infos s'affichent ensuite en dessous ainsi que l'impact de chaque joueurs. Tu vois quelle caractéristique nécessitant un "surentrainement" est à privilégier pour chaque style.
L'impact positif que tu as dans le style choisi est donc négatif pour l'adversaire et vice-versa.

I didn't mean what you understood. For example, I want to buy a player at auction for the "counter-attacking" style of play, so I need to know his impact to decide whether I want to buy him or not. For example: your player S. Youn, when he plays "take the lane", his impact in the counter-attacking style is -0.6.

If the stat doesn't impact the overall rating given the instructions / position, bonus gained should be calculated by stat level - 1 times (*) 0.657, I think (highest bonus is 64.4 for stat of 99, so 64.4 divided by 98 is approximately 0.657). I'm not that sure about this, and don't really fee like testing, but that's my opinion.
When the stat impacts overall rating, it probably has to do with the best point distribution for highest overall rating and the difference between that and the distribution, but I've never bothered with that. I have two central defenders with passing 99, but when instructed to participate in build-up, one gives slightly higher bonus (42.1) than the other (41.7) for short passing style.

Andre Malles: If the stat doesn't impact the overall rating given the instructions / position, bonus gained should be calculated by stat level - 1 times (*) 0.657, I think (highest bonus is 64.4 for stat of 99, so 64.4 divided by 98 is approximately 0.657). I'm not that sure about this, and don't really fee like testing, but that's my opinion.
When the stat impacts overall rating, it probably has to do with the best point distribution for highest overall rating and the difference between that and the distribu

Nice point of view, yes I agree with you on the instructions and the position but you forgot the skills and the effect differs if any of them differ because they are linked together, from my point of view the relationship between the style the player was trained on is inverse with the effect, for example you have a DC player who was trained on the style of participating in construction according to the best distribution of points his effect will be negative even though the passing skill is only trained in participating in construction so its value is negative but if we look at normal and mountain in stages that stopped not trained with the passing skill the effect value will be positive and equal in value due to the style reward but I think that the highest effect for the player is approximately 49.5 and the reason that one of your DC players is superior to the other even though they have the same value of the passing skill which is 99 is that one of the players was trained on the skills of defense, tackles and power more than the other because these skills reduce the player’s effect for short passes

The game first determines the player's overall rating based on the template linked to their individual instructions.
After calculating the overall rating, any remaining points from stats not used by the template, or overflow points, are then distributed as follows:

If the manager uses a style, the points are added as a bonus to that style at approximately 0.64 style points per point.

If the opponent uses a style that your template leaves points vulnerable to, those points are used against him at approximately 0.5 style points per point.

Teddy: The game first determines the player's overall rating based on the template linked to their individual instructions.
After calculating the overall rating, any remaining points from stats not used by the template, or overflow points, are then distributed as follows:

If the manager uses a style, the points are added as a bonus to that style at approximately 0.64 points style per point.

If the opponent uses a style that your template leaves points vulnerable to, those points are used against

I am talking about knowing the effect of the player’s instructions for a specific style, not the effect of the style on the statistics during the match. For example, you have a match and you play with a short-pass style and the effect of the style is +50, so your midfield statistics increase by 50 points and the opponent’s midfield statistics decrease by 50 points. What I am looking to know is knowing the value of the effect that the player will give when using a style. For example, you have a DL player M. Potter when you change his instructions to take the wing to a counter-attack style, this player will give an effect of -15 to be added to the total effect of the style.
So what I want to know is how did this value (-15) come to your player? I currently know how this value came to the Dl position for the Take the wing instructions for the counter attack style only.

In this case, the change in instructions leads to a decrease in your player's overall rating. However, you gain short-pass style points. These style points impact the overall statistics of your squad, not the player individually.

What you see reflected in the player's rating is their global rating minus the drop due to the loss in physical attributes.

In Potter's case, he drops to a 60 overall rating when set to "Take the wing," but contributes +20 to the short-pass style. So overall, you're getting the equivalent of an 80-rated player, which makes this example not very meaningful.

Does that answer your question?