The role of the Hero determines where they spawn at the beginning of a mission. There are currently 3 type of roles:

Front-line - Spawns closes to the enemies and is generally a tank.

Mid-line - Spawns in the middle of your side of the battle field is generally well-rounded Hero who focused on damage-per-second

Support - Spawns in the rear line and supports your other Heroes. 

There are only so many positions per map, so please note that if your team configuration contains too many of one type of Hero, they may be placed in odd positions on the map. Example: a team of five Front-line heroes will see some of those Heroes in long-range support positions.