It doesn't really mean anything - it's just a model number designation.
I'm not that familiar with the OM366 but the OM352 and 352A come in approx 11 configurations (7 for the normally aspirated and 4 for the turbo). Each 'dot number' designates a different variation - HP, ect.
OM352
353.902 2550 RPM 62KW 210Nm (rated RPM, HP, Torque)
353.907 2800 RPM 92KW 348Nm
353.940 2800 RPM 81KW 314Nm
353.980 2800 RPM 81KW 314Nm (must be some internal difference from .940)
353.949 2600 RPM 81KW 363Nm
353.961 2800 RPM 96KW 363Nm
353.962 2600 RPM 70KW 295Nm
OM352A
353.957 2600 RPM 92KW 392Nm turbo
353.973 2600 RPM 92KW 392Nm turbo (internal difference from .957 ??)
353.958 2600 RPM 110KW 461Nm turbo 16:1 comp (others 17:1)
353.959 2800 RPM 124KW 471Nm turbo 16:1
ALL M-B engines, gas and compression ignition, use that same model system.