A NULL-Canvas item is an item that has its Canvas property left unspecified, and so is not assigned to a canvas. A NULL-canvas item is never displayed at runtime, and operators cannot see the item or interact with it.
You can use a NULL-canvas item for temporary storage of data values, much like a global variable. For example, you can define an item of type NUMBER in a control block, and then reference its value in triggers and user-named subprograms:
IF :control.count > 200 THEN
...
Any type of item that can store values can be used as a NULL-canvas item variable. Display items are a good choice because they use less memory than other types of items.
Consider the following factors when deciding whether to use a global or NULL-canvas item as a variable: