Value formulas for computed fields
If the field type is Computed, Computed for display, or Computed when composed, you must write a formula to specify the field contents. This formula executes: for Computed, when the document is created and when it is saved; for Computed for display, whenever the document is opened; for Computed when composed, only when the document is created. Its value cannot be changed.
A computed field formula must evaluate to a value suitable for storage in the current field.