▁storage . ▁ ▁Use ▁case : ▁A ▁description ▁of ▁sequences ▁of ▁events ▁that , ▁taken ▁together , ▁lead ▁to ▁a ▁system ▁doing ▁something ▁useful . ▁Each ▁use ▁case ▁provides ▁one ▁or ▁more ▁scenarios ▁that ▁convey ▁how ▁the ▁system ▁should ▁interact ▁with ▁the ▁users ▁called ▁actors ▁to ▁achieve ▁a ▁specific ▁business ▁goal ▁or ▁function . ▁Use ▁case ▁actors ▁may ▁be ▁end ▁users ▁or ▁other ▁systems . ▁In ▁many ▁circumstances ▁use ▁cases ▁are ▁further ▁elabor ated ▁into ▁use ▁case ▁diag rams . ▁Use ▁case ▁diag rams ▁are ▁used ▁to ▁identify ▁the ▁actor ▁( users ▁or ▁other ▁systems ) ▁and ▁the ▁processes ▁they ▁perform . ▁ ▁System ▁sequence ▁diagram : ▁A ▁system ▁sequence ▁diag rams ▁( SS D ) ▁is ▁a ▁picture ▁that ▁shows , ▁for ▁a ▁particular ▁scenario ▁of ▁a ▁use ▁case , ▁the ▁events ▁that ▁external ▁actors ▁generate , ▁their ▁order , ▁and ▁possible ▁inter - system ▁events . ▁ ▁User ▁interface ▁document ations ▁( if ▁applicable ): ▁Document ▁that ▁shows ▁and ▁describes ▁the ▁look ▁and ▁feel ▁of ▁the ▁end ▁product ' s ▁user ▁interface . ▁It ▁is ▁not ▁mand atory ▁to ▁have ▁this , ▁but ▁it ▁helps ▁to ▁visual ize ▁the ▁end - product ▁and ▁therefore ▁helps ▁the ▁designer . ▁ ▁Rel ational ▁data ▁model ▁( if ▁applicable ): ▁A ▁data ▁model ▁is ▁an ▁abstract ▁model ▁that ▁describes ▁how ▁data ▁is ▁represented ▁and ▁used . ▁If ▁an ▁object ▁database ▁is ▁not ▁used , ▁the ▁rel ational ▁data ▁model ▁should ▁usually ▁be ▁created ▁before ▁the ▁design , ▁since ▁the ▁strategy ▁chosen ▁for ▁object -