The Cursor is the mapmaker's best friend when making a custom map. It is used in Debug Mode to select trigger areas and open a block's UI.
Cursor Hotbar Icon
In Debug Mode, to use the Cursor and make selections, it must be held in a player's 'right' hand.
If the player is close enough to a block to see it's outline, then they can use the Cursor to left-click it to select the block as a coordinate. To create a 1x1 trigger area, click again on the same block. To select a larger trigger area, click on that first block, then click on a different block. The southeastern-most block in this trigger area is considered the "Min" of the coordinates, while the northwestern-most block is the "Max".
To open a block's UI, right-click it with the Cursor. Right-clicking is also used to set the teleport coordinates for a Teleport block.