Summary:
Lookup Control settings determine how data is populated from a lookup field. There are four control options, each defining how and when values are applied when using a lookup on the Redesign Screen. Below is a description of each control and its behavior.
Resolution:
Control = 1
Updates and auto-populates the value every time the record is created or edited.
Control = 2
Sets the value only if the field is currently blank.
Control = 3
Sets the value only if the field is currently blank or zero.
Control = 4
Sets the value only if the lookup field is not blank.