In the United States, for example, a subsidiary letter guaranteeing employer neutrality in union elections in newly acquired factories, subsidiaries or departments led to federal action on the legality of the agreement in 2002 and, in 2007, to an important decision by the National Labor Relations Board to revise federal labor policy.   In Australia, side letters are becoming more frequent due to changes in federal labour law made by the WorkChoices Act. WorkChoices limits the collective agreements that parties can register for the application of labour courts and also requires that ACAs be strictly limited to labour-related matters. The inclusion of minor clauses not related to the workplace (e.g. B the statement of royalties) may lead to the inapplicability of a CBA. In response, many unions and employers use side letters to reach agreement on non-labour issues and do not register these secondary letters with the federal government – and rely on customary law to enforce the ancillary letters.  Annex 3 to the Ancillary Agreement can only be amended by a written agreement between the parties. Under contract law, a subsidiary letter has the same force as the underlying contract.  However, courts may invalidate secondary letters in conflict with the master collective agreement.  The provisions of the CBA govern the interpretation of lateral letters . . .