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 only details which are not restricted to measuring instruments or to any other apparatus covered by a single class. This subclass does not cover: details covered by any other subclass in section A, F, G or H. In particular, details restricted to the measuring instruments are covered by the relevant subclasses of class , e.g. ; constructional details restricted to electric apparatus, e.g. casings, screenings, which are covered by subclass or the relevant subclass in section H. Attention is drawn to the Notes following the title of section G, especially as regards to the definition of the term "measuring" in Note (2) following the title of class . SENSITIVE ELEMENTS PRODUCING MOVEMENT OR DISPLACEMENT; DETAILS OF MOVEMENTS ; ADJUSTING POSITION OR ATTITUDE; COMPENSATING FOR TEMPERATURE EFFECTS ; HOUSING, SUPPORTING; INDICATING ELEMENTS ; CALIBRATING COOLING; SCREENING ; DETAILS OF APPARATUS USING SCANNING-PROBE TECHNIQUES This group covers: the protection of instruments or other apparatus from external radiation or other influences; the prevention of the emission of undesirable radiation or other influences by instruments or other apparatus. In this group the following expression is used with the meaning indicated: "probe" means an interfacing device, e.g. a needle with a sharp tip, defining a transducing zone where an interaction, e.g. generation of a tunnelling current when in close proximity to a surface, is supported. Attention is drawn to the Notes following the titles of class and subclass relating to "micro-structural devices" and "micro-structural systems" and the Notes following the title of subclass relating to "nano-structures". The following is a list of places where specific applications using scanning-probe techniques are provided for: Manufacture or treatment of nano-structures Surface treatment of metallic material Dimensional measurement Investigating or analysing surface structures in atomic ranges , , Recording or reproducing information Discharge tubes with provision for introducing objects or material to be exposed to the discharge Processes or apparatus adapted for the treatment of semiconductors or solid state devices