In a blueprint book you can place blueprints in arbitrary points of the grid of the book. Say you put one blueprint in the first place and another third place, leaving the second position empty, then you're unable to deploy the second blueprint, because if you send the signal = 3, the deployer puts the first blueprint only (probably because the book has 2 blueprints, so it assumes that 3 is wrong), if you send the signal = 2, then nothing happens, because the second position has no blueprint.
If the assumtions I made above are correct, then the meaning of the construction signal seems inconsistent, is it the number of blueprint or the number of the position in the book?
