• D-descriptors, which define the data actually stored in a field
• I-descriptors, which are calculated fields, evaluated whenever the value is required
• On UniData systems, V-descriptors (which define virtual fields) are like I-descriptors

I-descriptors can perform calculations on data stored in one record, or retrieve data from other files. For example, records in the CUSTOMER file have a field that lists related record IDs in the ORDERS file. The CUSTOMER file dictionary could contain I-descriptors that use the TRANS function to retrieve fields from those related records.