Does the experience shown in the player sheet represent the number of years the players played in the league (+ the time he was expected to have played when the player is generated in "young" leagues) or does it represent only an ability (faster familiarity with team playbook/opponent scheme, slower injury recovery, higher probability of retirement,...)?

It seems to me that some players gets their experience upped in the middle of the season which seems to me as odd if it is the first case and misleading if it is the second case.

"Experience" is time played in the position. So if your player is in that position during a game, he will gain experience for that position. So yes, experience will change during the course of the season; increasing if he plays the position more. Not sure if there's any decrease if he's played in a different position, but... That can be answered by someone else.

There is only decrease for positions he is not assigned to. If he still plays some downs at that position it will probably counter that decrease. He does increase during TC and during the week's practices for the position he is assigned to in addition to his playing time.

Thank you...

I was actually not refering to the position experience rating but to "experience" mentioned in the details of the players (between his age and his general rating).

That is the number of years he has been in the league.

EDIT: sorry double posts

Thank you, do you confirm it has no purpose in the game (for exemple, to determine the retirement probability only age matters)?

Well, in this case, that's a little bit surprising that the number of year got incremented in the middle of a season... maybe I was wrong on my recollection of the experience of some of my players.

There is very little range between the player's age and his experience, so the values are almost the same in terms of retirement probability. The number should not change midseason, though.