The Master File describes the following field characteristics:

• The name of the field, as identified in the FIELDNAME attribute.
• Another name for the field, either its original name, as defined to its native data management system, or (for some types of data sources) a synonym of your own choosing, or (in some special cases) a pre-defined value that tells how to interpret the field, that you can use as an alternative name in requests. This alternative name is defined by the ALIAS attribute.
• How the field stores and displays data, specified by the ACTUAL, USAGE, and MISSING attributes.

  The ACTUAL attribute describes the type and length of the data as it is actually stored in the data source. For example, a field might be alphanumeric and 15 characters in length. Note that FOCUS data sources do not use the ACTUAL attribute, and instead use the USAGE attribute to describe the data as it is formatted. FOCUS handles the storage.

  The USAGE attribute, also known by its alias, FORMAT, describes how a field is formatted when it appears in reports. You can also specify edit options, such as date formats, floating dollar signs, and zero suppression.

  The MISSING attribute enables null values to be entered into and read from a field in data sources that support null data, such as FOCUS data sources and most relational data sources.

• The option for a field to be virtual, rather than being stored in the data source, and have its value derived from information already in the data source. Virtual fields are specified by the DEFINE attribute.
• Optional field documentation for the developer, contained in the DESCRIPTION attribute.
• Acceptable data-entry values for the field, specified by the ACCEPT attribute.
• Online help information about the field that an end user can display during an application, as described by the HELPMESSAGE attribute.
• An alternative report column title for the field, described by the TITLE attribute.
• A 100-year window that assigns a century value to a two-digit year stored in the field. Two attributes define this window: DEFCENT and YRTHRESH. See the Developing Applications manual.