The plugin first checks the total weight and volume [ if volume calculation is enabled in the backend] of the products in cart
1. if ( total weight > sendle's max weight OR total volume > sendle's max volume)
2. if yes then it distribute the cart products in to multiple package like
if the cart has 4 products with weight=20 lbs and volume=100 in3 each then it will divide it into
3x20=60 lbs in one package and 1x20=20 lbs in another package.
IN US the sendle's max weight limit is 70 lbs and max volume limit is 864 in3
so the 1st package weight = 60 lbs and volume=300in3
and 2nd package weight = 20 lbs and volume=100in3
3. Now in the above example if the cart products volume is 400in3 each with the same weight
then the 1st package volume would be 1200in3 which is greater than the sendle's max volume limit.
in this case the plugin will distribute the cart again according to the volume like
1st package volume = 2x400 = 800in3 and weight= 40 lbs
and 2 package volume = 2x400 = 800in3 and weight= 40 lbs
The weight is mandatory for the sendle rate calculation but the volume is not.
That's why we first try to distribute it into multiple packages according to weight and then if any package volume is greater than the max volume then it will distribute it again into multiple packages according to volume.