The umpires assigned to a green should either set the 2-metre marks on the rinks before the start of a game, or check their accuracy if the marks have been set by the greenskeeper. Once these marks have been set or confirmed by the umpire, they must be accepted as correct, with no right of appeal by any player regarding the placement of the mark. When a jack is re-spotted, it is to be placed at the 2-metre mark.
[...] [T]he only possible situation that may arise which questions the accuracy of a 2-metre mark would be when the mat has been placed at the 2-metre mark, a short jack is delivered, and an umpire is called to determine if the jack has been improperly delivered. Umpires are trained to measure the distance of the mat line from the rear ditch to ensure that it is a minimum of 2 metres from the ditch. It is possible that the 2-metre mark could have been set in error less than 2 metres from the ditch. If the mat line measurement is less than 2 metres, and the 2-metre mark is also determined to be less than 2 metres, then the 2-metre mark should be corrected immediately before the end proceeds any further.
Many clubs are using a nail to mark 2 metres. This is acceptable. However, we have updated the guidance on how the nail should be placed. BCB advises:
The use of a nail is permitted, but the entire nail head should also be beyond the 2-metre line, so that the jack can be aligned with the back edge of the nail head. The nail head is smaller than the “T,” is round instead of rectangular, and we know that the nail head can easily be removed and relocated, so it does have some shortcomings.
You can review the updated memo here. The updated diagram is below.