U can use the others too, but no more than 1 relation between 2 activities.
So the relation between act.A en act.B can be either one of FS,SS,FF,or SF but not SS and FF.
In a true forward&backward network analysis to determine Early and Late dates this would result in open ends on the activity , if only SS ( open finish ) or only FF (open start) was used. The combination of SS and FF ( possible in P3) will prevent this.