27.This Section sets out the definitions of telecommunications and postal services and systems relevant to the Act, and assists in the interpretation of interception and other related matters. For the interpretation of other terms used in Chapter I of Part I, see sections 20 and 81.
“Private telecommunication system” is defined as any telecommunication system which is not a public telecommunication system; but is attached to such a system.  This means that an office network, linked to a public telecommunication system by a private exchange, is to be treated as a private system.  Interception of such a system other than by the system controller or with his consent is a criminal offence.  An entirely self-standing system, on the other hand, such as a secure office intranet, does not fall within the definition.
28.Subsection (2) explains what constitutes the interception of a communication in the course of its transmission by means of a telecommunication system. This is relevant to the criminal offence and the civil liability in Section 1; and to the issuing of a warrant by the Secretary of State which authorises or requires interception in Section 5. There is no equivalent definition for postal interception.
Commentary on Sections "International mutual assistance agreement" is defined in Section 20 Section 2: Meaning and location of “interception” etc