C = Center, D = Driver's side, P = Passenger side
2D = 2nd row Driver's Side. 3D = 3rd row Driver's side.
2C = 2nd row Center seat. 3C = 3rd row Center seat.
2P = 2nd row Passenger seat. 3P = 3rd row Passenger seat.
HR = Head Restraint (also called a head rest)
Overlap is where the seat belt (either the belt or the buckle) belonging to one seat is located in its neighboring seat's territory. The 2015-17 Nissan Rogue (image below) is an example of egregious overlapping of the driver & center seats - where half of the driver & center seats are purple indicating where they overlap.
In the 2015-17 Nissan Rogue, the driver's side seat belt buckle is nearly 5 inches into the center seat... or another way of saying it is that the center seat belt is 5 inches into the driver's side seat. Either way, 2 car seats can't occupy this same 5 inches of shared space (purple area in the far right image), which means that in this vehicle you can not use the driver's side seat (for an adult or car seat) if you put a car seat in the center and you can't use the center seat (for an adult or car seat) if you put a car seat on the driver's side. It is important to note that there are many vehicles with less severe overlap than the Rogue where it is possible with certain combinations of car seats to get either 2 car seats side by side, or an adult + car seat side-by-side.
Overlap is an issue that affects not only a seat belt installation, but also a LATCH installation for a car seat - as the overlapping seat belt often crosses over a lower anchor you were hoping to use. In addition, 2 car seats can't sit on top of each other... so 1 car seat overlapping into the space needed for the adjacent car seat can prevent you from getting 2 car seats securely installed side by side.
If you plan to put anything - car seat or booster - or anyone in the center of your back seat you'll want to know whether we were able to fit a narrow booster in the center. We've found that a narrow booster is a good surrogate for judging whether the center of that back seat is likely to accommodate a car seat or stand a chance at fitting an adult's bottom. So even if you don't have plans to put a backless booster in the center you'll still want to know if that backless booster can fit because it will tell you if the vehicle's center is likely to be a usable spot for a car seat or an adult's bottom or not. In cases where the narrow booster did not fit in the center, it is best to think of this center as a wasted spot in the car. In cases where the narrow booster did fit in the center, you can be comfortable knowing that you will be able to find some car seats & boosters that will be able to fit in the center - and that skinnier adults should also fit nicely.
We used the Harmony Youth Booster as it is an exceptionally narrow backless booster - and at less than $15 is within anyone's budget. This booster serves as a proxy; if the Harmony fits in the center, then the center is wide enough to accommodate at least one booster (the Harmony) and typically some of the car seats that have very narrow footprints. It is important to keep in mind that just because the Harmony fits in the center of a particular vehicle, it does not mean that you will be able to fit another car seat next to the Harmony - sometimes you may, sometimes you may not.
If you are exceptionally tight on space, the Boostapak is an exceptionally narrow backless booster you may want to try instead of the Harmony - as while both are similarly narrow at the footprint, the Harmony flares out at the arm rests but the Boostapak keeps its slim profile with arm rests that don't flare out. 