A domain is a pool of values from which users can choose when providing values for a type of attribute. A type of attribute may be stored in multiple columns. These columns are said to be based on the domain.
A domain is a rich data type. In addition to delimiting a pool of values, it also specifies other key aspects of columns based on the domain. For example, it may specify in which format the Rules Engine is to present stored date/time values to the user, or which type of visual control must be used in the screen for columns based on the domain.
A domain containing all-uppercase alphabetic strings with a maximum length of 60, such as 'SMITH', could be used for attributes that are customer names.
A domain containing integers in the range 0-100 could be used for attributes that are percentages. The domain could specify that these integers must be shown in a Slider control that allows users to "spin" quickly to the desired value.