In Fibreboard, the Supreme Court held that, based on its three-part analysis, an employer`s decision to allocate part of its business to subcontractors was a mandatory subject of negotiation. First, subcontracting is within the literal meaning of the NRA`s term «terms and conditions of employment.» Second, to find that subcontracting is a subject of compulsory bargaining has the effect of achieving the objectives of the NRA by bringing «a problem of vital interest for work and management within the framework defined by Congress as the most conducive to peace at work», namely collective bargaining. Third, other employers in the same sector have referred to the awarding of contracts as part of the bargaining process instead of leaving it to management`s discretion. Secondly, there are a number of restrictions on autonomy arising from compliance with the provisions of constitutional law and general law, a general restriction that stems from the hierarchy of legal sources that the law itself establishes by stipulating that collective agreements must not limit the exercise of fundamental rights guaranteed by the Constitution or lead to binding legislation. The ban on trade union membership (Closed Shop) is a consequence of this. It is also prohibited for agreements to infringe the legal provisions on minimum conditions of work and employment; Only provisions more favourable to workers are permitted, whether introduced by individual autonomy or, as a general rule, by collective autonomy. It should be noted, however, that in some cases the legislation prohibits the establishment of systems of regulation through collective autonomy, different from those provided for by law, and, in other cases, prohibits the provision of more favourable conditions for workers (e.g.B. immediately after the revolution, but still in the 1989 Dismissal Act). Portuguese labour law contains various such restrictions. . . .