The firstPartyHosts property is used during the initialization of the RUM React Native SDK to identify which network requests are made to your own backend services. For any request matching a host in this list, the SDK automatically adds Datadog tracing headers. This allows the backend APM agent to link the server-side trace with the originating frontend user action, creating a complete distributed trace from the client to the backend.

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1. Datadog Documentation, "React Native Monitoring," Connect RUM and Traces. (https://docs.datadoghq.com/realusermonitoring/reactnative/datacollected/connectrumandtraces/). This page explicitly states: "To link RUM data with traces, you need to specify the hosts of your first party requests in the firstPartyHosts array during the SDK initialization."

2. Datadog Github Repository, dd-sdk-react-native, "Advanced Usage" documentation. The README.md file in the repository details the firstPartyHosts configuration option for enabling distributed tracing.