Sometimes the rules will call for a unit to enter the battle. Where this happens, we refer to the unit entering play as reinforcements.
Units that enter the battle (also referred to as moving onto the board) as reinforcements are placed in base contact with the board edge (upon which board edge, and where, will depend upon the rule that triggers the reinforcements) facing directly towards the battlefield and with all of its rear rank touching the battlefield edge.
A unit that enters as reinforcements cannot charge, as it has missed its opportunity to declare charges, and may not march, but can otherwise participate in the game normally. It's worth bearing in mind that the unit counts as having moved for the purposes of shooting. In addition, all the models in the unit must be set up so that they are within twice their Movement rate of the battlefield edge that they entered from.