Difference between SULF and SO2 in SMOKE

I am curious what the difference between species SULF and SO2 is in SMOKE emissions files, since by default, in DESID, SULF from SMOKE maps to 0 as CMAQ gas species and maps fully to CMAQ fine aerosol species ASO4 while SO2 from SMOKE fully maps to CMAQ gas species SO2:

&EmissionScalingRules
 EM_NML=
 ! Region      | Stream Label  |Emission | CMAQ-        |Phase/|Scale |Basis |Op  
 !  Label      |               |Surrogate| Species      |Mode  |Factor|      |     
       ! Default Gases                                                                  
   'EVERYWHERE', 'ALL'         ,'SO2'    ,'SO2'         ,'GAS'  ,1.  ,'UNIT','a', 
   'EVERYWHERE', 'ALL'         ,'SULF'   ,'SULF'        ,'GAS'  ,0.  ,'UNIT','a', 

   ! Default Aerosols
   'EVERYWHERE', 'ALL'         ,'SULF'   ,'ASO4'        ,'FINE' ,1.  ,'MASS','a', 

When I do ‘ncdump’ of SMOKE emission files, I see the following description for SULF and SO2, and I couldn’t distinguish SULF from SO2:

    float SULF(TSTEP, LAY, ROW, COL) ;
            SULF:long_name = "SULF            " ;
            SULF:units = "moles/s         " ;
            SULF:var_desc = "Model species SULF                                                            " ;

    float SO2(TSTEP, LAY, ROW, COL) ;
            SO2:long_name = "SO2             " ;
            SO2:units = "moles/s         " ;
            SO2:var_desc = "Model species SO2                                                               " ;

SULF represents “Sulfate as SO4”. SO2 and SULF are different. SULF (SO4) molecular weight is 96 grams.
Not all cases, some sources use the following the SO2 speciation profiles to generate SO2 and SULF during the speciation processing in SMOKE. Some limited sources will generate an additional 2.86% of SO2 emissions as SULF.
75014;“SO2”;“SULF”;0.0286;98.0;0.0286
75014;“SO2”;“SO2”;1.0;64.0;1.0
87514;“SO2”;“SULF”;0.0245;98.0;0.0245
87514;“SO2”;“SO2”;1.0;64.0;1.0
87514;“SOX”;“SULF”;0.0245;98.0;0.0245
87514;“SOX”;“SO2”;1.0;64.0;1.0
95014;“SO2”;“SULF”;0.0226;98.0;0.0226
95014;“SO2”;“SO2”;1.0;64.0;1.0
95014;“SOX”;“SULF”;0.0226;98.0;0.0226
95014;“SOX”;“SO2”;1.0;64.0;1.0
99010;“SO2”;“SULF”;0.0155;98.0;0.0155
99010;“SO2”;“SO2”;1.0;64.0;1.0
99010;“SOX”;“SULF”;0.0155;98.0;0.0155
99010;“SOX”;“SO2”;1.0;64.0;1.0
0000;“SO2”;“SO2”;1.0;64.0;1.0
0000;“SO4”;“SULF”;1.0;96.0;1.0
0000;“SOX”;“SO2”;1.0;64.0;1.0