Object and memo fields have unlimited size. Every record in a database must have one or more key fields. One of the key fields must be unique. This is called the primary key. Key fields are used for retrieving information.
Some databases use multiple fields to give a unique primary key.
In order to retrieve records you need to know how they are stored. Direct access by hashing Sequential files are stored sequentially. There can be more than one index to an ISAM file. The index key for this file must be unique. Batch processing is usually used in accounting systems.