What a platform data source is
A Tealium data source connects a specific system - a SaaS platform, a cloud warehouse, a webhook, a file feed - to the server-side layer. "Platform data sources" are the pre-built connectors for common systems; a generic webhook or File Import source is what you configure by hand when no pre-built connector exists.
When you need one
Any time server-side collection needs to originate from something other than a client-side tag on a webpage - a CRM firing a webhook on a status change, a cloud warehouse holding an attribute worth syncing, a partner system pushing conversion events directly.
How to set one up correctly
Pick the source type deliberately - a pre-built platform connector when one exists is almost always less work and less risk than a hand-built webhook doing the same job. Authenticate per that source's actual requirement (API key, OAuth, signed request - this varies by source and by Tealium release, so confirm current requirements rather than assuming). Define the mapping from the source's native fields to Tealium events or attributes explicitly; do not accept a default mapping without reading what it actually does. Decide the downstream destination before going live - a specific EventStream, a data layer attribute, an AudienceStream visitor attribute - and set access controls to match who should be able to see or edit the source.
How to verify it worked
Trigger or send a real test payload from the source system itself, not a synthetic one built to look convenient. Confirm the trace or log on the Tealium side shows a received event with the fields you expect, and confirm the mapped fields actually populate on a test profile - not merely that the source shows a "connected" status.
Common mistake: treating "connected" as the finish line. A data source that authenticates successfully but maps the wrong field, or none at all, will report healthy indefinitely while quietly producing nothing useful.