A child car seat restraint can be placed and installed in a side seat when you cannot put the car seat in the middle seat.
For example if you have only one back seat and have more than one child in a car seat you can put one of the car seats in a side seat and it will be just fine.
Preferably it's best to put the car seat in the middle seat instead of the side seat near a door but it's okay to put a car seat in the side seat.
Also if you have only a front seat such as a truck that has no back seat it's okay to put the car seat in the front seat but in the middle is the best place to put it.
But if you have to put the child car seat near the door it's okay but also make sure to turn off any airbags in the front when a child is in the car seat.
Or any child 12 or under that is riding in the front seat should have the air bag turned off because the airbag can injure or kill a child 12 or under.
The back seat is always the safest seat for any kid 12 or under.