[For other posts about the 2020 rules and changes to the rules, see THIS.]
The 2020 version of the FIPJP rules contains new rules about where the jack may be thrown or placed. This has prompted players to ask questions like this one on Ask the Umpire.
The rules say that the jack must be thrown on the assigned terrain (see Article 6). So the question essentially boils down to this—
What does “on the assigned terrain” mean? Is a thrown jack straddling a side guide line considered to be “on the assigned terrain”?
The answer to this question was provided by Mike Pegg. The thrown jack must be touching the ground of the assigned lane.
A jack that is touching the ground inside the line is valid even if it is also touching the the line (string) around the assigned lane. Because the jack is larger than the string, if the jack is touching the string, it will also be slightly bulging over the string. That’s OK. The jack is valid.
The rules are similar, but not exactly the same, for the throwing circle. To be considered “on the assigned lane”, the entire circle must be inside the boundary lines of the lane. The side of the circle may touch the line, but the circle cannot extend over the line.