You've probably heard of the LATCH system for installing carseats. LATCH is an acronym that stands for
Lower
Anchors and
Tethers for
CHildren.
Most vehicles 2002 and newer will have the LATCH system in at least two rear seating positions. There are exceptions, things like large trucks or vehicles that are classified as large trucks (Ford Excursion is one that comes to mind).
The lower anchors will be located in your vehicle seat bight (seat crease), the top tether anchor locations will vary by vehicle, but in a sedan, generally they are located in your rear window deck.
A rear-facing seat will only be installed with either the seatbelt or lower anchors (never use both methods together), not with the top tether anchor, unless you have a Britax (I see you do
) or a Sunshine Kids Radian. These seats do allow top tethering when rear-facing, but you don't use the standard top tether anchor for rear-facing tethering.
Your Fusion owner's manual will indicate which rear seating positions have LATCH. It will be your choice to install with lower anchors or with the seatbelt. Use whichever method gets you the best install. If both methods are equally tight, use whichever method is easiest for you. Using the top tether is optional, but encouraged. The Britax Marathon owner's manual will indicate how to use the d-ring in order to tether rear-facing, and there's many threads here talking about it, too.
So basically, the term LATCH refers to the entire system of lower anchors plus top tether anchors, the term lower anchors means just the anchors in the seat crease, the term top tether anchor refers to just the top tether anchor (which is where you hook a forward-facing seat's top tether strap).
You'll use either lower anchors or seatbelt for a rear-facing install, and you may choose to use the rear-facing top tether or not. When your child is big enough to be forward-facing, you'll install using either the lower anchors or seatbelt, but you'll absolutely want to top tether it to the top tether anchor, no question (at least 1 year old
and at least 20 pounds is the bare minimum to go forward-facing, but it's safest to rear-face to the limits of the seat, which is 35 pounds rear-facing, or until the head is within an inch of the top of the seat shell).
I know the different terms can be confusing at first, hopefully we cleared it up for you! If not, don't hesitate to post back.