# IUPAC numbering of carbon atoms in a chain

A chemistry handout for nomenclature of org. compounds I've been reading states a particular rule for every functional group viz.

If there is a choice in numbering not previously covered, the parent chain is numbered to give the substituents the lowest number at the first point of difference.

Can anyone explain what it means?

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.