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 class covers: simulators which are concerned with the mathematics of computing the existing or anticipated conditions within the real device or system; simulators which demonstrate, by means involving computing, the function of apparatus or of a system, if no provision exists elsewhere; image data processing or generation. This class does not cover: control functions derived from simulators, in general, which are covered by class , although such functions may be covered by the subclass of this class for the device controlled; measurement or analysis of an individual variable to serve as an input to a simulator, which is covered by class ; simulators regarded as teaching or training devices which is the case if they give perceptible sensations having a likeness to the sensations a student would experience in reality in response to actions taken by him. Such simulators are covered by class ; components of simulators, if identical with real devices or machines, which are covered by the relevant subclass for these devices or machines (and not by class ). In this class, the following terms or expressions are used with the meanings indicated: "data" is used as the synonym of "information". Therefore, the term "information" is not used in subclasses , or ; "calculating or computing" includes, inter alia, operations on numerical values and on data expressed in numerical form. Of these terms "computing" is used throughout the class; "computation" is derived from this interpretation of "computing". In the French language the term "calcul" will serve for either term; "simulator" is a device which may use the same time scale as the real device or operate on an expanded or compressed time scale. In interpreting this term models of real devices to reduced or expanded scales are not regarded as simulators; "record carrier" means a body, such as a cylinder, disc, card, tape, or wire, capable of permanently holding information, which can be read-off by a sensing element movable relative to the recorded information. Attention is drawn to the Notes following the title of section G, especially as regards the definition of the term "variable". In this subclass, the following expression is used with the meaning indicated: "hybrid computing arrangement" is an arrangement in which part of the computation is digital and part is analogue.