In addition to using FlowFile attributes, you can also define custom properties for Expression Language use. Defining custom properties gives you additional flexibility in processing and configuring dataflows. For example, you can refer to custom properties for connection, server, and service properties. Once you have created custom properties, you can identify their location in the nifi.variable.registry.properties field in the 'nifi.properties' file. After you have updated the 'nifi.properties' file and restarted NiFi, you are able to use custom properties as needed.