Function execution timeliness essentially refers to whether or not the operations performed by Screen are immediate or delayed.
Most of the Screen API functions aren't executed immediately. Instead, the commands resulting from the function calls are queued at the API for batch processing later in time. As your application makes multiple API function calls, commands accumulate in a command buffer that's associated with a context. These commands are batch-processed either when the command buffer is full, or when an API function of the type flushing execution is called. By batch-processing these commands, a large number of commands can be submitted in one atomic operation and the communication between the client and the Composition Manager is reduced to fewer, larger messages.
However, it's important to note that functions that are executed immediately may additionally flush a set of queued commands. Furthermore, a function that flushes any queued commands may or may not necessarily cause a redraw of the display.
You should understand the exact execution type of each API function. Knowing how and when these functions will be executed in the scope of your Screen application will be fundamental to your development.