Voluntary (or permissible) bargaining items are neither mandatory nor illegal; they become a part of negotiations only through the joint agreement of both management and union. Neither party can compel the other to negotiate over voluntary items. It cannot hold up signing a contract because the other party refuses to bargain on a voluntary item. Benefits for retirees might be an example.
Voluntary Bargaining Items are items in collective bargaining over which bargaining is neither illegal nor mandatory--neither party can be compelled against its wishes to negotiate over those items.