1. A method of generating a logical field specification for a logical field associated with a physical entity of data in a computer system, the physical entity of data having a particular physical data representation, the method comprising: providing a logical field specification template comprising a plurality of specification sub-fields, each specification sub-field designated by a sub-field descriptor; accessing the physical entity of data to determine data items associated with the sub-field descriptors; and linking each specification sub-field to a corresponding determined data item.