Here is what the IBM Information Center has to say about that:
Use IPL type D when directed for special work, such as installing and reloading programs. IPL type D loads the system programs from an alternate IPL load source, such as a tape drive or CD-ROM. Typically an IPL uses programs that are stored on the primary IPL load source (typically a disk drive). Sometimes it is necessary to perform an IPL from another source, such as programs that are stored on tape. To do this, you must use IPL type D to perform an IPL from the alternate IPL load source.
Use IPL type D only during one of the following situations:
- When the installation or restore procedures direct you to use IPL type D
- When IPL type B and IPL type A fail (when the primary IPL load source cannot perform an IPL on the system properly) and only when directed by your support personnel
- When service directs you to perform an alternate installation
You can specify the type of initial program load (IPL) you need to perform from the system control panel. The Increment/Decrement buttons are used to change the IPL type and mode on systems without a mode button. Use Function 02 to select the IPL type (A, B, or D) and mode (normal or manual). To select IPL type and mode from the control panel, follow these steps:
- Use the Increment/Decrement buttons to select Function 02, and press Enter.
- Use the Increment/Decrement buttons to select the IPL type and mode you want, and then press the Enter button to save.
This was first published in January 2009