The IsLoadingScreen function is a built-in function that returns True only when invoked within a Web Screen Preparation execution and only if the screen is loading its content. Otherwise, if the screen is just being redrawn, i.e. posted back, it returns False.
A web screen is loading when it's being entered from:
The IsLoadingScreen function only applies to Web Screens. When called in any other action (SMS, Web Services, and Timers actions), it returns False.
The following action flow is the Preparation flow of the Edit Customer screen; the Customer record is fetched from the database only when the screen is being loaded (fresh start):
Screen Preparation | Available Miscellaneous Functions | Available Built-in Functions