When the procedure in run it will do the following:
Test the current location for a specific block.
Remove the block at the current location.
Place a new block at the current location.
You can use ( y: [Y] +  ) to off set the ordinates to test for repetitive positive locations. This would test for one block above the block
You can set the coordinates variables to just numbers to test of static locations in the world ( x:  y:  z:  ) will test for a block at (0, 1, 0) in the provided world.