If the extra hole is a foot or so above the Tack (forward low corner of sail) it's for the cunningham. A cunningham is a very handy mainsail control feature that helps move the sail's draft position forward or aft. If the hole is higher, and.... there is a corresponding hole on the "leech" (back edge of sail) then it's a reef point. Reef points on mainsails, with one exception, always appear in pairs. Reefing a sail reduces its area, depowering it. Okay, NOW is the time to invest in a basic saiing handbook. I suggest "The A'nnapolis Book of Seamanship"OK back, here is a photo
On some masts the gate for getting the slugs into the track is fairly high. When reefing the slugs stop at the gate with a stopper or they will fall out of the track. If the slugs stop at the gate there will be several inches of sail that can not be brought down to the boom for a proper reef. The jacklines allow the slugs to stop at the top of the gate while allowing for the sail itself to drop to the boom.. I cannot remember what the reason was for this arrangement (partial photo attached).