Many new runners make the mistake of thinking SS means the run needs to be completed with the highest possible ranking. This is not what SS means. SS stands for Single Segment. All full game runs are SS. Segmented is only for individual level runs.

   Segmented - The runner is allowed to restart any checkpoint as much as they'd like to for any reason (Non-tactical restarts are allowed). This allows the chapter to be beaten much faster. The game will "save" your IGT at the start of each checkpoint, allowing the runner to try the checkpoint over and over without actually losing time.

   SS (Single Segment) - Only tactical resets are allowed. Basically any reset used to fix a mistake will invalidate the run. There are various reasons for a runner to reset that dont involve fixing a mistake. These include despawning enemies, managing your inventory, and spawning Chris closer to Sheva or Sheva closer Chris.  

 There are more guides on tactical and non-tactical reset that will also be helpful for new runners. I suggest looking at these as well if your unfamiliar with the RE5 speedrun.