PIE_ON contains details about what PIE is used for and you should first of all refer to that.
Presuming that PIE is enabled (with PIE_ON), you may want to treat any programs (or toolkits) which use SD.EXTOP machine code calls to access the screen differently.
Normally, the Window Manager halts any program which attempts to call the SD.EXTOP machine code routine unless that program does not have any buried windows. However, PIE_ON allows all buried programs to continue in the background, storing the changes to the screen as necessary.
However, SD.EXTOP routines may be used for various purposes including writing to the screen directly and for this reason, if PIE is active, you may find that a program writes to the screen using SD.EXTOP even though its windows are buried (thus overwriting part of an existing program’s display).
PIE_EX_OFF prevents this effect by still halting any program which attempts to use SD.EXTOP.