While not an expert I did some work with fleet replacement of aircraft seats a couple of years ago. The design criteria for large civil aircraft are laid down in CS-25. From memory CS25.561 & 562 are applicable. Most seats (within EASA) are approved under the ETSO system; each approved item is given a unique number.
Essentially, a seat should be designed so that when subjected to certain loads it will not deform to an extent were it might impede a rapid exit of the occupants from the aircraft cabin. The test loads are upward, forward, sideways, downward & rearward, varying between 1.5 and 9g. These are conducted used an anthropomorphic dummy weighing 77kgs. Specific parts of the dummy are subject to loads during these tests. Assuming your seat is a passenger seat fitted with a lap belt, the load to be considered is the pelvis. The maximum compressive load should not exceed 680kg, and therefore my guess is that this is the figure you have been quoted.