SS is StarShell.
Standard ammo types:
HE - High Explosive - blast damage, good against crew and soft targets, but doesn't make holes (penetrate)
AP - Armor Piercing - used against hard targets, but only puts holes in it - no boom-boom
AA - Anti-Aircraft - like it says - time-fused with a small blast charge (FLAK)
SS - Star Shell - illumination used to light up a target at night or other low light conditions
There is a debate on exactly what each Uboat actually carried - out of approx 220 rds of 105mm on U 199 there were 40 AA rds - see crew interrogation here:
http://www.uboatarchive.net/U-199INT.htm