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A complicated item may have a high number of required functions. All functions are not equally important, and a classification may therefore be an aid for identification and analysis purposes. One way of classifying functions is as follows:

 Essential functions. These are the functions required to fulfill the intended purpose of the item. The essential functions are simply the reasons for installing or using the item. The essential function is sometimes reflected in the name of the item. An essential function of a pump is, for example, to “pump fluid.”

 Auxiliary functions. These are the functions that are required to support the essential functions. The auxiliary functions are usually less obvious than the essential functions, but may in many cases be as important as the essential functions. Failure of an auxiliary function may in many cases be more safety‐critical than a failure of an essential function. An auxiliary function of a pump is, for example, to “contain fluid.”

 Protective functions. These functions are intended to protect people, equipment, and the environment from damage and injury. The protective functions may be classified as follows:Safety functions (i.e. to prevent hazardous events and/or to reduce consequences to people, material assets, and the environment)Security functions (i.e. to prevent vulnerabilities, physical attacks, and cyberattacks)Environment functions (e.g. anti‐pollution functions)Hygiene functions (e.g. for items used in food production or in hospitals).

 Information functions. These functions cover condition monitoring, various gauges and alarms, communication monitoring, and so forth.

 Interface functions. These functions apply to the interfaces between the item in question and other items. The interfaces may be active or passive. A passive interface is, for example, present when the item is a support or a base for another item.

 Superfluous functions. These functions are never used and are often found in electronic equipment that have a wide range of “nice to have” functions that are not really necessary. Superfluous functions may further be found in systems that have been modified several times. Superfluous functions may also be present when the item has been designed for an operating context that is different from the actual operating context. In some cases, failure of a superfluous function may cause failure of other functions.

Some functions may belong to more than one class. For some applications, it may further be relevant to classify functions as follows:

1 Online functions. These functions are operated either continuously or so often that the user has current knowledge about their status. The termination of an online function is called an evident or detected failure.

2 Off‐line functions. These functions are used intermittently or so infrequently that their availability is not known by the user without some special check or test. Some offline functions are not possible to test without damaging the item. An example of an offline function is the essential function of the airbag system of a car. Many protective functions are offline functions. The termination of the ability to perform an offline function is called a hidden or undetected failure.