In this section, the following term is used with the meaning indicated: "variable" (as a noun) means a feature or property (e.g., a dimension, a physical condition such as temperature, a quality such as density or colour) which, in respect of a particular entity (e.g., an object, a quantity of a substance, a beam of light) and at a particular instant, is capable of being measured; the variable may change, so that its numerical expression may assume different values at different times, in different conditions or in individual cases, but may be constant in respect of a particular entity in certain conditions or for practical purposes (e.g., the length of a bar may be regarded as constant for many purposes). Attention is drawn to the definitions of terms or expressions used, appearing in the notes of several of the classes in this section, in particular those of "measuring" in class and "control" and "regulation" in class . Classification in this section may present more difficulty than in other sections, because the distinction between different fields of use rests to a considerable extent on differences in the intention of the user rather than on any constructional differences or differences in the manner of use, and because the subjects dealt with are often in effect systems or combinations, which have features or parts in common, rather than "things", which are readily distinguishable as a whole. For example, information (e.g., a set of figures) may be displayed for the purpose of education or advertising (), for enabling the result of a measurement to be known (), for signalling the information to a distant point or for giving information which has been signalled from a distant point (). The words used to describe the purpose depend on features that may be irrelevant to the form of the apparatus concerned, for example, such features as the desired effect on the person who sees the display, or whether the display is controlled from a remote point. Again, a device which responds to some change in a condition, e.g., in the pressure of a fluid, may be used, without modification of the device itself, to give information about the pressure () or about some other condition linked to the pressure (another subclass of class , e.g., for temperature), to make a record of the pressure or of its occurrence (), to give an alarm (), or to control another apparatus (). The classification scheme is intended to enable things of a similar nature (as indicated above) to be classified together. It is therefore particularly necessary for the real nature of any technical subject to be decided before it can be properly classified. This subclass covers also means for identifying or incapacitating burglars or the like. This subclass does not cover: the mere provision of an audible or visible signalling device on measuring or switching apparatus; alarm systems for indicating that a specific variable has exceeded, or fallen below, a predetermined value, which are covered by the relevant subclasses of class for the measurement of that variable. alarms for specific processes or types of machines or apparatus, which are covered by the relevant subclasses for the processes, machines, or apparatus. In this subclass, the following term is used with the meaning indicated: "systems" may cover also devices peculiar thereto. SIGNALLING OR CALLING SYSTEMS Characterised by the transmission of the signal Characterised by the nature of the indication: audible; visible; tactile; combined ; ; ; ORDER TELEGRAPHS ALARM SYSTEMS Responsive to an unspecified condition Responsive to two or more different conditions Responsive to one specified condition: intrusion; fire; other , ; ; With transmission from or to a central station , , Predictive alarm systems CHECKING, MONITORING TRANSMISSION SYSTEMS IN GENERAL Electric; non-electric ; SYSTEMS FOR TRANSMITTING THE POSITION OF AN OBJECT ARRANGEMENTS CHARACTERISED BY THE METHOD OF TRANSMISSION Multiplex; use of a wireless electrical link ; PROCESSING SIGNALS Differentiating, delaying MONITORING, PREVENTING OR CORRECTING ERRORS This subclass covers: identification of traffic offenders; indicating the position of vehicles for traffic control purposes; navigation systems for traffic control purposes, i.e. systems in which the navigation is not performed autonomously by or in the vehicles, but where the vehicles are guided by instructions transmitted to them; indication of free spaces in parking areas.