In some scenarios, forces have had time to conceal their troops and prepare booby traps and minefields to delay the enemy. When using Hidden Set-up you will need a hidden set-up marker for every unit in your force (including those in reserve or not starting on the tabletop for some other reason).
When forces are deployed on the table the defender places hidden set-up markers instead of models. A marker may be placed on any terrain, as long as it can be moved over by the model it is representing, and is in the defender's normal deployment zone.
Each marker needs to be numbered so that you can note down which number corresponds to which unit – if you don't have any numbered markers simply make your own by tearing off some pieces of scrap paper and numbering them. Alternatively, the defender can draw a map of the tabletop and mark the location of each of his units on it.
See Hidden Vehicles, Revealing Hidden Troops and Minefields and Booby Traps for the rest of this rule.