When ValidateOnChange is true, this property is activated. When set true, when the user changes this field, AJAX code will be used to update the field at the host and tab/enter the field to see if there are any errors or messages/updates to inform the user of.
Note that this only has value in ASCII/Character mode hosts, as TN3270 and TN5250 screens are not updated live the way that VT220 and other character mode protocols.