meaning of retiring auditor

Can any one explain sec 140(4) of companies act 2013.i didn't understand the words ....... Except retiring auditor who completed him term..... Because any how they cannot reappoint.... Please could explain the above sec clearly

sec 140(4) specifies the situations where special notice is required as per sec 115

it is up to members in deciding to appoint another person than retiring auditor or, expressly providing not to appoint the retiring auditor

here retiring auditor simply means the auditor who has completed his tenure as the case may be u/s 139(2).

auditor to be reappointed need not be ratified by the members ( amendment act 2017). 

therefore sec140(4) applies where the auditor has not completed his tenure

Within the 5 years tenure of the auditor , at every AGM, auditor will become the retiring auditor ,OR if it completes its 5 year tenure ,then he becomes the retiring auditor?