A CTL -1017 is used to indicate that the input is to logically restart. That is the input position is to be placed at the first character of the input and if the next keyboard input is a printable character (non-edit key), the input will be cleared and a new input accepted. If the next keyboard input is a edit key (arrow, Insert, Delete, Home, End, etc...) the system will start the input edit process.
This CTL value is normally generated by the RSTART key as defined in the terminal definitions however it is also generated internally by ProvideX/PxPlus by an INPUT EDIT directive.
When an INPUT EDIT directive is executed the system will load the input buffer with the current value of the variable then insert a CTL -1017 into the input queue.