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: recording or playback of information by relative movement between a record track and a transducer, the transducer directly producing, or being directly actuated by, modulation in the track being recorded or played-back, and the extent of modulation corresponding to the signal being recorded or played-back; apparatus and machines for recording or playback, and parts thereof, such as heads; record carriers for use with such apparatus and machines; associated working of other apparatus with such apparatus and machines. In this subclass, the following terms or expressions are used with the meanings indicated: "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 relatively to the record carrier; "head" includes any means for converting sinusoidal or non-sinusoidal electric wave-forms into variations of the physical condition of at least the adjacent surface of the record carrier, or vice versa; "near-field interaction" means a very short distance interaction using scanning-probe techniques, e.g. quasi- contact or evanescent contact between head and record carrier. Attention is drawn to the Notes following the titles of class and subclass relating to "micro-structural devices" and "micro-structural systems". RECORDING OF ONE TYPE ASSOCIATED WITH REPRODUCING MEANS OF THE SAME TYPE Of mechanical type Of magnetical type Of optical type Of another type RECORDING OF ONE TYPE AND ASSOCIATED REPRODUCING MEANS OF DIFFERENT TYPE SIMULTANEOUS OR SELECTIVE RECORDING OF DIFFERENT TYPES; ASSOCIATED SIMULTANEOUS OR SELECTIVE REPRODUCING MEANS SIGNAL PROCESSING NOT SPECIFIC TO THE METHOD OF RECORDING OR REPRODUCING APPARATUS CHARACTERISED BY THE SHAPE OF THE RECORD CARRIER DETAILS; GENERAL FEATURES Starting, stopping, driving , Guiding HEADS; RECORD CARRIERS ; ASSOCIATED WORKING WITH OTHER APPARATUS EDITING, INDEXING, SYNCHRONISING, MONITORING MANUFACTURING , , OTHER CONSTRUCTIONAL PARTS, DETAILS OR ACCESSORIES Groups take precedence over groups . This group does not cover compositions, materials or processes, per se, which are covered by the relevant subclasses of section B or C. In group , multi-aspect classification is applied, so that if subject matter is characterised by aspects covered by more than one of its subgroups, the subject matter should be classified in each of those subgroups. Group takes precedence over groups Group takes precedence over groups . This group covers arrangements in which there are at least two recordings of information involving two different methods or means or two different physical properties, at the same or different locations, on the same record carrier, the recordings being made or reproduced simultaneously or selectively. Where such combinations of means are used for changing only one main property, classification is only made in one of the relevant main groups , , , or . Group takes precedence over groups . In group , recording or reproducing apparatus does not include the record carriers.