The "DeIsL1.isu" file is only present on a Windows machine after installing 4D, e.g.:

Path--> C:/program files/4D/DeIsL1.isu

This file contains the InstallShield's uninstall instructions for a particular program. When a user uses the uninstall wizard, the wizard checks the DelsL1.isu file for specific instructions and does the uninstall according to those instructions.