hey upto my knowledge these rules are used in assigning configuration to optically active compunds

for example

the  most oxidised group comes on top of your fischer projection

hydrogen preferrable at bottom

and for assigning the config

more atomic number higher the priority

eg br>o>c etc

and after sorting them count from 1 to 4 in the case of carbon compounds and if clockwise assign R if H is in the vertical and S if H is in horizontal

the opp if anti clockwise

 

between C2H5 and CH3 C2H5 is given more preferrence!!!

 

now understood ah???