IUPAC numbering of carbon atoms in a chain
Consider a n-hexane compound

$\ce{CH3-CH2-CH2-CHCl-CH2-CH3}$

The Cl is different than the H of n-hexane. 

Going left to right the Cl is on the 4th carbon. Going right to left it is on the 3rd carbon. So you should use the right to left numbering in this case.