These orders are nothing to do with the price but they are completely based on timing.
1) Pre Market Order:- Orders placed during the pre-opening market session that is from 9 a.m. to 9:15 a.m. is called as pre-opening market orders.
Orders placed in this session will be executed only during the market hours.
2) Market Order:- Orders placed during the market hours i.e 9:15 a.m. to 3:30 p.m. are called as market orders.
3) Post Market Order(PMO):- Orders placed during the post-market hours i.e 3:30 p.m. to 4 p.m. are called as PMO.
This session is not for the retail individual investors but only for brokers.
4) After Market Order (AMO):- Orders placed after the market hours i.e from 4 p.m. to 9 a.m. the next day in the morning are called as AMO.
We may place AMO’s at any time after the market hours but they will get executed at 9:15 a.m. when the market opens. AMO’s also affect the price of shares at the opening and it is rarely possible that opening price of today is equals to closing price of yesterday.