O horizon

Thanks to the explanation but
1- Can we say O horizon differ with Oi , Oe, ,
2- if Oi horizon ( 2 cm depth) presents ,so , 0 -2 is correct or 2-0
example : we have 80 cm of one profile with horizons including Oi ,A,B,C
Can we write 2-0 ,0-20 ,20-50 , 50-80, or
0-2 ,2-20 20-50 , 50-80,
which is correct?