It has a margin for error in it but they basically take the closest frame from where the foot makes contact with the ball to set the point where they then judge. There is a margin for error in this.
The VAR then draws the line as closely as possible to where the relevant body parts are then then technology adjusts the line to factor in the camera angle and make it 3D.
I just think the issue is they’re trying to be mm accurate with technology that can’t be mm accurate.
Also, the rule was made stricter to factor in the error in the human eye.
They’ve now decided that VAR doesn’t judge offside as a clear and obvious error they judge it as black and white so every goal has to be checked by the technology and as far as the league is concerned the technology is always right as it is consistent in giving the right call but it’s still programmed by a human being and where the line is drawn could be wrong so it isn’t perfect.
The issue for me is currently the technology takes to long to get to a final decision in close calls and it doesn’t really reflect the spirit of the law. They changed the handball law because of VAR my guess is they will now change the offside law as well as they aren’t going to get rid of VAR.
For me until the technology is good enough to give a quick accurate decision they should go back to judging the offside by eye and only ruling out clear and obvious errors which is what people were sold VAR as doing.