An InstallShield Product Code is a unique identifier (a GUID) used by the Windows Installer to distinguish a specific version of a software application. Key Characteristics
If you are a system administrator trying to automate a deployment, you can find the Product Code of an already installed app using PowerShell: powershell installshield product code
Warning: Never reuse a Product Code from one product on a completely different product. This will confuse Windows Installer, causing it to think the new product is already installed or leading to corrupted registry entries. An InstallShield Product Code is a unique identifier