The DISPLAY variable-list TO field-list or DISPLAY BY NAME variable-list instruction fills the value buffers of specified form fields with the values contained in the specified program variables. The DISPLAY instruction changes the buffers directly in the current form, not the program variables bound to the dialog. DISPLAY can be used outside any dialog instruction, in the same way as the CLEAR instruction. DISPLAY also sets the modification flag of fields.
As DIALOG is typically used with the UNBUFFERED mode, there is no reason to set field buffers in a DIALOG block since any variable assignment will synchronize field buffers. Actually, changing the field buffers with the DISPLAY or CLEAR instruction will have no visual effect if the fields are used by a dialog working in UNBUFFERED mode, because the variables bound to the dialog will be used to reset the field buffer just before giving control back to the user. So if you want to set field values, just assign the variables and the fields will be synchronized. However, when using a CONSTRUCT binding, you may want to set field buffers with this DISPLAY instruction, as there are no program variables bound to fields (with CONSTRUCT, only one string variable is bound to hold the SQL condition).
Instead of using a DISPLAY instruction to set the modification flag of fields to simulate user input, use the DIALOG.setFieldTouched() method instead.