Set it to false if you do not use the full Bill of Lading functionality and you want N4 to treat the BL number as only a text field.
The default value for INVBL001 is true.
You can use the Shipment Details form to record the BL number for a unit. In addition, N4 displays the BL number in the Units view and in the Status area in the Unit Inspector.
For validating the POD of the unit against the POD of the BL, see the CGOCargoValidation002 (MATCH_BL_UNIT_POD) (on page 1) setting.