Field edit can be used to locally check the syntactial errrors made by end user( mostly). It can be used for conplex checking operations too but rarely.

Save edit is used to check the logical existance or invalid datta inputs.It can be used for conplex checking operations.

Example if a broker becomes ineffective after a perticualr date and if some calculations are done after that date logic can be written in save edits to stop further processing and help the user identify another broker whose effictive date is valid for the date specified.