• with return value
• control functions (with void return value)

The functions with return value are standard functions that occur in expressions. On the basis of their evaluation, they gain the value that is then implied into the expression evaluation.

Control functions (with a void return value) are functions that may not occur in an expression, because they do not gain the value. Their evaluation is characterized by the execution of the specific operation, on the basis of the function parameters. Such functions may be used only in a script, not in the expression, e.g. for object of I/O tag type.

All up-to-now implemented control functions are used only in the context of ESL script within the frame of Active picture.

The control function in ESL script, are written like other functions. This is the example of notation of the control function %HI_VisiblePicture, that hides or shows the picture according to one value of its one parameter.