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Property names can be custom or defined by plugins. For example, to replace WebLogic installation folder you can use ${{FDWLS_WEBLOGIC_HOME}} which is defined by FlexDeploy WebLogic plugin. But if you want to replace something like URL to internal service, you can define something like MYCOMPANY_PAYMENT_SERVICE_URL on workflow as defined in Creating a Workflow and use it in your file as ${{MYCOMPANY_PAYMENT_SERVICE_URL}}. Defining custom properties are entirely up to your implementation. For example, you can just define some prefix or suffix that changes through environments as a custom property instead of defining entire URL value.
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