I personally think this is one of the most confusing aspects of the Laws of the Game. In particular, I'm referring to these two paragraphs (Law 11.4):
(Defending players off the field of play)
A defending player who leaves the field of play without the referee’s permission shall be considered to be on the goal line or touchline for the purposes of offside until the next stoppage in play or until the defending team has played the ball towards the halfway line and it is outside its penalty area. If the player left the field of play deliberately, the player must be cautioned when the ball is next out of play.
(Attacking players off the field of play)
An attacking player may step or stay off the field of play not to be involved in active play. If the player re-enters from the goal line and becomes involved in play before the next stoppage in play or the defending team has played the ball towards the halfway line and it is outside its penalty area, the player shall be considered to be positioned on the goal line for the purposes of offside. A player who deliberately leaves the field of play and re-enters without the referee’s permission and is not penalised for offside and gains an advantage must be cautioned.
I actually have the same two questions for both paragraphs, so they're technically four questions (just in case one or both of these questions have different answers depending on whether we're talking about defending or attacking players being off the field of play):
- is a defending/attacking player still considered to be on a boundary line even after he has re-entered the field of play?
(This first question stems from the fact that neither paragraph says that re-entering the field of play actually stops this virtual-repositioning-on-the-boundary-lines that otherwise happens for the purposes of offside.
I'm especially curious about this for what concerns attacking players, as this seems to imply that an attacking players who had stepped off the field of play and then got back will still be considered to be a "ghost player" in offside position even if five minutes pass and the ball has been kicked around multiple times, as long as play never stopped, and the ball never left the penalty area while being kicked towards the halfway line.)
- do these paragraphs apply to defending/attacking players who leave the field of play as part of a playing movement as well?
(This second question stems from the fact that both paragraphs seem to be about very specific situations, as the first one is about players leaving the field of play without the referee's permission, while the second one is about attacking players leaving for the purposes of not being involved in active play.
While I understand that if a player leaves the field of play with the referee's permission, e.g. because of an injury, it's only natural that he shouldn't be considered for the purposes of offside anymore, Law 11 never touches the important situation of a player leaving the field of play as part of a playing movement as per Law 3.8, so that I'm unsure about what the referee should do when e.g. an attacking player temporarily leaves the field of play to go past an opponent, or when e.g. a defending player accidentally steps behind their own goal line while defending their goal).