relation PWB Sapscript or smartforms

Hello

1)  im trying to use pwb  and my questionis ,  are there sapscript or smartfprms   already assigned  to each PWB CLASS

     or shold we assign one by ourselves ?

2) if they do exists (sapcript ot smartforms)  How can we find each corresponding one easily ?

3) if we modify the sapscript should we do it thru  application form only ?

thanks a lot

Tahar