If no rectification interval (A,B,C,D) is stated then the item must be serviceable. As required by the authorities comes about due to the MMEL being provided for operators to create there own MEL from which could be in any country where the jurisdiction may vary slightly. i.e EU Ops v FAA regs and potentially small differences between the part 25 requirements between the US and EU (if any) the dash is always applicable it just depends on the operator. This is for both old style MELs (since rectification intervals) and New.
The dash in other columns is also applicable for qty installed & required when the qty varies amongst the fleet.
The preamble in the front section of the MEL should explain also.