Consideration must be given to compliance, for example, with legal or other regulations or policies and compatibility, for example, with existing systems. Only essential requirements should be included and as far as possible they should be expressed in terms of interfaces. Explicit technical specifications may be required to define the limits of compatibility or to describe an item that has already been designed or has been prescribed. For example, the communications systems to be provided by the private sector on a private road must be compatible with the relevant government agency's own systems.