The Mason-Dixon line was flown at Fort Delaware in the middle of Delaware river for a few minutes in honor of those that fell and suffered during the US Civil War. Fort Delaware was a POW camp where many captured Confederate soldiers ended up after the battle of Gettysburg.
After it was flown, a small explanation to the staff was given about is meaning as a direct line of latitude at 39º 43', in decimal form is about 39.71666º. It does not follow the slave soil vs free soil boundary, rather the North-South Calvert-Penn flag does that, which you can see here. Rather is cuts across the US from New Jersey, Delaware, Pennsylvania/Maryland, West Virginia, Ohio, Indiana, Illinois, Missouri, Kansas, Colorado, Utah, Nevada, and California. It also represents persons who have ancestors who fought on both sides of the War Between the States.
Not only that this flag is meat for all Americans and anyone tied to tragedy and hope of the US Civil War, this war cut its deepest scars into the heart of America be it black, white, or brown.