The system creates a window data structure for each window, which includes extra space that an application can use to store additional data about a window. An application specifies the number of extra bytes to allocate in the WinRegisterClass function. All windows of the same class have the same amount of window data space.

An application can store window data in a window's data structure by using the WinSetWindowUShort and WinSetWindowULong functions. It can retrieve data by using the WinQueryWindowUShort and WinQueryWindowULong functions.