PDI is the name of the SCO UnixWare 1 and 2 driver interface for storage devices. It is the predecessor of the UnixWare 7 SDI. PDI drivers that conform to $interface sdi: 2 or higher and ddi: 5 or higher can be run on UnixWare 7 without modification.
Administrative commands have been renamed (for example, the pdiadd command is now sdiadd(1M)), with links provided so that scripts that use the old name will function without modification.