When you have a line with posting key 40 for 1000 and created a new line with 50 and a start. Sap will fill in 1000. When you use 40 then a * is not possible.
Use this option not for postings from users. Whe the had key in wrong 1100 the posting is wrong, there is then no control from the debit and credit posting for the reason that you post the diffrences with a *.