Particle forces within particlesys allow greater control of the dynamic beahvior. You can create general effects such as wind and gravity within the prt-script itself. However for location based effects you must add particle forces with this class. You can also get access to the orginal prt scripts. The number of total forces per particlesys is limited to 32 and name must be shorter than 16 characters. The more you add, the slower it will be.
o-+ ressubid o-+ particleforceid