It can be used for prevention also. You would use TIG (or antitoxin) in a person with a serious (high risk) wound who is not fully immunized, or in any AIDS patient with a tetanus-prone wound regardless of immunization status. Tetanus immunization can be given at the same time.
Serious=wounds contaminated with dirt, soil, feces, saliva, puncture wounds, or those with devitalized tissue, e.g. avulsions, from missiles, crushing, frostbite, burns.
Information is from the Red Book.