The bags have the safe working load labeled on the side of the bag, as we can see from this bag the SWL is 1000kg. This bag has a safety factor of 5:1 meaning that it will fail at 5 times its SWL. Most often these bags are used for delivery of sand, blue metal, gravel or soil to site. Weights vary but for dry sand they usually weigh 900kg, blue metal is around the same, weights increase if wet. They can be a convenient way of spreading material around by cutting the base of the bag when it is lifted with the crane and moving to the desired area.
There are some very important things to keep in mind when lifting these, ALWAYS USE ALL FOUR LOOPS. I have seen lazy dogmen try to only hook up two loops, for this they should be kicked off site! Also if the bags have been in the sun, even just a week, they start to break down, loosing a great deal of strength. If you lift a bag that has been in the sun for a while there is a good chance the loops will fail, causing the bag to fall and this can cause serious injury and property damage.