Scenario – A custom field has been created with Store Value unchecked(maybe formula field or sourcing from saved search). This custom field value needs to be sourced to another custom field. But we can not source value from a field with Store Value unchecked.
Solution – Create the first custom field without checking the Store Value box so that the formula value populates the custom field.
To source this value to the second custom field, use a workflow.
Workflow action – Set field value
Field – Second Custom field(with store value checked)
From field – First field(with store value unchecked)
Select appropriate trigger type and context