An ARTIS model comprises eight elements:

• a diagram with groups and items, this determines the impact of capacity fluctuations and downtime of the groups and items on the system
• a list of units, with downtime modes and maintenance and inspection strategies of the items
• a configuration form, for user preferences
• data interfaces, for up- and download
• expressions, for all integer and real number input
• file operations, for opening and saving models
• a network definition interface
• runs, on any selection of the diagram

An item is either on the production or on the consumption side. Apart from the side, production and consumption items have the same input parameters. The consumption items can have capacity profiles, planned downtime, unplanned downtime, and other parameters, like the production items. The consumption items define the demand, so they set the context for the production items. Most of the production availability results are relative to the demand.

For a consumption item, its demand is defined as its expected capacity, in the usual way, that is by taking account of the planned and unplanned downtime.

For a production item, no demand is given.

A model, and any selection, has given demand, if one of the following situations:

• a series has given demand if and only if at least one of its elements has given demand
• a parallel arrangement has demand if all its elements have given demand
• a network has given demand if and only if all paths from source to sink have given demand.

For a model, or any selection, that doesn't have a given demand according to the above rules, the demand is assumed to default to its peak capacity. This convention makes the consumption modelling consistent with all previous ARTIS versions, standard reliability block diagram terminology, and with the ISO 14224 and ISO 20185 definitions.