Subaru 2004 Forester Owner's Manual page 97

Seat, seatbelt and SRS airbags
There are many types of collisions which might not
necessarily require SRS side airbag deployment. In
the event of accidents like those illustrated, the SRS
side airbag may not deploy depending on the level of
accident forces involved.
1-70
When will the SRS side airbag not deploy?
1
2
1) The vehicle is involved in frontal collision with another
vehicle (moving or stationary).
2) The vehicle is struck from behind.
US1569BB