Semantics specify what the different characters mean. In this case, the semantics are summarised in Figure 3.12. The notion of codes is used here as an overall concept
1. Nonfigurative codes, which consist of several small elements. The individual elements have no meaning in themselves, but when combined they have common unambiguous meanings. The most common form of these nonfigurative codes is numbers and letters.
2. There are also figurative codes (symbols), which are those designed in such a way as to have a meaning themselves without needing to be combined with other symbols. Such figurative codes may be either concrete or abstract. The concrete codes attempt to imitate what they symbolise (e. g., a pedestrian crossing sign is represented by a stylised drawing of a walking person), while the abstract codes symbolise an abstract concept (e. g., ‘Christianity’, represented by a cross).
Both figurative and nonfigurative codes are used in process industry applications. The figurative concrete codes should try to resemble the apparatus and machines they represent, and the abstract codes should be used to represent the actual events occurring during the process. Nonfigurative codes, usually in the form of letter and number abbreviations, are also used. Numbers are used both for identification and quantification although it is preferable to use two different number series for these purposes, for example, Roman numerals for identification and Arabic (ordinary) numbers for quantification.