SLS is the Self Leveling Suspension, and that bottle is the resevoir for the suspension fluid. Be sure to use only the MB Hydraulic fluid for leveling systems as anything other may cause damage to a very expensive pump or other suspension parts (struts and accumulators).
Judging from the picture it appears the system is at the proper level of fluid, or mighty darn close.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.