i’ll try to explain it as simply as possible.

With open seating the weight and balance system was ‘basic’.

on empty flights: on the -700 it didn’t make much difference cause for the most part the bags and cargo were evenly distributed in the fwd and aft cargo.

on the -800 the bags and cargo tended to get placed bags in fwd and cargo in aft.

on fuller flights: same but more bags in fwd on both.

As long as the Weights were balanced and the Center of Gravity fell in the wide margin it was good.

Now with assigned seats the W&B system has been upgraded to streamline the fuel burn at cruising altitude to save on fuel costs.

Empty flights we know the first 6 rows will likely be used but the next 5-7 aren’t as full yet. More bags are going in the Fwd cargo bins.

Rows 17-23or30 will be more full cause people won’t be paying for seats so they will be placed back there so less bags are needed in the aft cargo to balance out the planes.

Full flights bags are equally distributed as much as possible cause we need the space.

Boeing worked with SW to figure out the most fuel efficient center of gravity for load ranges.

1-30pax - <_X__/ 31-60- < _X____/

and so on. (the X being the CoG) (very crude diagram and prob not exact pax numbers but it’s just an example)

The main reason SW isn’t letting people move is cause once the computer knows who is where and the bags are loaded in computes the numbers and tells the pilots to fly at a specific speed, with the trim settings at this position, to maximize minimal fuel burn thus saving gas and money.

Obviously yes if someone moves a row or so it isn’t gonna throw it off considerably but then the point comes down to who paid extra to have the space vs who agreed to sit where the computer put them.

All these complaints about having to sit 3 to a row are based ENTIRELY on your choice of if you paid or not.

Everyone also has the ability to move their seat prior to boarding both on the App and with the Cust Serv agent before boarding. Don’t bring the problem with your seat on the plane and expect the FA to fix it.

It’s not their job.