The form and content of any classification reflects its purpose, and it is recognized that no one classification can meet all needs. The World Health organization has developed the concept of a "family of classifications" built around the core of the ICD itself (1992). The "peripheral" members of the family would include specially based adaptations of ICD, (e.g. for psychiatry, oncology, dentistry and stomatology), the International Nomenclature of Diseases (the purpose of which is to provide a single recommended name for every disease entity), and other health-related classifications such as the International Classification of Impairments, Disabilities and Handicaps (ICIDH). It is an explicit goal of the ICNP project that ICNP should join the "family," probably among the "Other Health Related Classifications."

It is already clear that the ICNP cannot serve all the purposes that all nurses in every country would like. Its explicit purpose, as stated in the Alpha Version (ICN, 1996), is to "provide a vocabulary, a new classification for nursing, and a framework into which existing vocabularies and classifications can be cross-mapped to enable comparison of nursing data collected using other recognized nursing vocabularies and classifications." Even these three purposes are difficult to achieve by means of a single tool.

Meerabeau et al (1997) point out that nurses already use different languages for different purposes. They note that the U.S. National Center for Nursing Research (1993), distinguishes between clinical terms (the language of practice) and definitional terms (the language of nursing knowledge — theory and research). Hoy (1995) sets out a continuum of steps between "informal language" which nurses use to communicate information about patients whose care they share, and the "formal language" which is necessary for remote communication such as anonymised aggregated data for research or statistical purposes. It has been suggested that as nurses learn to articulate more precisely their phenomena of concern, the gap between the two extremes of Hoy's continuum will narrow, but it is unlikely that they will ever merge.

Classification brings even greater problems. The existing nursing classifications, like the ICD, are first-generation mono-axial classifications, and have usually been inductively developed. The ICNP Alpha Version classification of nursing phenomena was also mono-axial, but it differs from the other nursing classifications in that it is built according to strict rules of classification based on generic relations — that is, the concepts are arranged in a strict hierarchy in which each subordinate term is related to each superordinate term (the genus) by a principle of division, and distinguished from other terms on the same level by its special characteristics (the characteristic of the species).

The meaning of the concept is, therefore, defined by its place in the classification as well as by any other definition it may be given. The ICNP Beta version will use multi-axial classification in which each complex concept (e.g. impaired mobility) is broken down into separate axes, (e.g. mobility : impaired). This kind of classification increases richness and flexibility because it allows the terms in different axes to be combined in various ways, but the penalty is that the increased complexity limits use to computerized systems, to which nurses in many countries have no access.