Mandatory fields on several tabs: I have a question concerning the handling of mandatory fields in InputScripts. Suppose I move a mandatory field from an inactive tab to the main tab by creating a GuiXT input field. How do we cope with the error handling if a user enters a wrong value in creation mode? In change mode it is possible to replace the wrong value with the previously valid one and then switch to the main tab and display an error message. But how can we handle this situation in creation mode?

As you said, this problem appears in "creation" mode, not in "change" mode, since you can set the previous field value and then leave this tab.

In creation mode, if you know a particular value that is a valid one in each case, you set this value into the mandatory field and return with the error message. Clearly you do not set the value into your V-variable for the InputField.
If you do not know a valid value for this field, you are not able to leave the tab in your InputScript. You can now either restart the whole transaction with /N... and keep all field values entered (set a special indicator variable for this case), or you display the tab with the mandatory field and wrong field input and let the user select a different value. This requires some programming work, since you need to work with the "Link" commands in order to  obtain the values the user entered into the SAP fields; and the user is probably astonished to see a different screen layout now.

By the way, mandatory fields on other tabs (not the first one) do not occur very often in SAP, since it is bad user interface design: if a user selects such a tab, she will not be able to leave it without finding valid values for all mandatory fields; same situation as in the InputScript.