NOTE : The LOOKUP expression will be recalculated on every save of the record containing the expression. If the value for the Field in the calling App matches multiple values in the Lookup App then the value brought back will be from the first record found.
AppID is the ID of the App being used to store the lookup data.
ThisAppFieldID is the ID of the Field in the calling App being used as the field to look up a value in the Lookup App
LOOKUPAppFieldID is the Field ID in the lookup App being compared to the Field selected in the calling App.
LOOKUPAppFieldToReturn is the Field for which the value will be returned on a successful find of a Record.
ValueIfNotFound is the value the Field is to be set to if no Records in the Lookup App are a match.
ExactMatch? is a boolean property. If true then the values in the compared Fields must be an exact match and if false then the first record in the Lookup App with a value for the compared Fields that contains the value from the calling app will be a match.