PalmOS
For much the same reason as desk accessories were used in Mac OS and in GEM, namely to allow more than one simultaneous program on a system that did not support multitasking, the concept of desk accessories was extended to PalmOS by third-party developers. DA are applets launched by an application or hack serving as a DA launcher. The DA launcher may watch for keystrokes or other system events and pop up a pre-defined desk accessory. Many general purpose PalmOS launcher applications are capable of launching DAs as well.
A desk accessory program is a Palm resource database of type 'DAcc', specified to include a single 'code' #1000 resource that contains the binary code implementing the desk accessory. Global or static variables are not available, but a DA can call user interface APIs. It is possible for a DA to have user interface resources in its database. The desk accessory launcher transfers execution to the first byte of the 'code' #1000 resource.
DAs provide a modicum of multitasking. However, unlike in Mac OS and GEM, after the user is done working with the DA, it must be closed to return to the underlying application. It is possible to pop a DA up over another DA, though this might deplete stack space.
Read more about this topic: Desk Accessory