The NHTSA (National Highway Traffic Safety Administration) has released new laws about car seat safety starting in 2014. The law in itself is a little difficult to understand, however the basic premise is: if your child's weight plus the weight of the car seat is over 65 lbs, then do NOT use the LATCH system of attaching the car seat to the car, you should use only the seat belt to attach the seat to the car according to the manufacture's instructions. The only exclusion is that if a top tether is available to use, you should continue using it with the seat belt, as this top tether will help keep the seat secure and limit movement in the event of a crash, but as always be sure to follow the instructions that came with your seat.
It is also important to note that you should never use both the LATCH system and the seat belt system at the same time because using both can cause failure in the event of a crash.