Any multi-form application involves at least two forms: a calling form and a called form. Parameters help a lot in passing data from the calling form to the called form, that is, in the forward direction. Sometimes there is a requirement to pass data from the called form to the calling form.
As an example, consider an employee-department screen that lists the employee information for each department in an organization, including each employee's address. Because the address information is display only, the address of an employee is changed by means of a link to a separate address screen. Therefore, the new address must be selected in the address screen (the called form) to be populated in the employee screen (the calling form) when the address screen is exited. This data transfer in the reverse direction, that is, passing data from the called form to the calling form, can be done using global variables only.
Use parameters for passing data across two forms in the forward direction and global variables for passing data in the reverse direction, unless the same data has to be shared across multiple forms. In this case, use global variables.
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