The item-ID and each attribute of the file-defining item contain information that points to and describes the lower level dictionary file or data file. Because it has a D dictionary code, the file-defining item is sometimes referred to as a D-pointer.
There are two file-defining item types:
System Dictionary, mds
File definition items define each master dictionary (account) on the system.
Master Dictionary, md
File definition items define data files.
In a file dictionary, file definition items define the file's data section.