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Yes—under the generic VAST 4.x XML Schema, an `` can contain a `` child and still pass XSD validation. That establishes structural validity, not compliance with IAB Tech Lab’s CTV signaling guidance. In that guidance, `pos` and `plcmt` are distinct signals, and the documented pattern pairs an extension’s type with its corresponding child. Treat the example as a signaling mismatch, and validate both the XML and the applicable CTV requirements.

Why the VAST schema can accept this XML

The IAB-maintained VAST 4.x XSD allows zero or more child elements inside ``. Its wildcard uses `processContents=”skip”`, so the schema does not validate the extension’s child content. The `type` attribute is an optional string; the schema does not require its value to match the child element’s name.

Consequently, the schema does not reject `` simply because its parent is ``. The document must still satisfy the other applicable VAST schema requirements. This conclusion concerns the generic VAST 4.x schema, not every VAST version or implementation.

Why schema acceptance does not mean the CTV signal is correct

IAB Tech Lab’s final Signaling Implementation Guidelines give these signals different jobs:

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  • plcmt identifies the placement subtype or format.
  • pos describes placement position or on-screen treatment.
  • playbackmethod describes how playback of the ad is initiated.

They are not interchangeable labels. Under the documented pattern, a `pos` extension carries a `pos` child, while a `plcmt` extension carries a `plcmt` child. Pairing `type=”pos”` with `` is therefore structurally admissible to the generic schema but mismatched with that CTV signaling pattern.

What downstream stitchers are expected to do

The guidance describes VAST as a way to round-trip `pos` and `attr` information so publishers, stitchers, and measurement vendors can validate the creative experience without access to the original bid-response payload. It says the patterns apply to both client-side rendering and server-side stitcher implementations.

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That states the intended contract; it does not establish how every deployed stitcher handles a mismatched extension. Without documentation or testing for a particular product, do not assume it will reject, ignore, or reinterpret the child based on either the `type` attribute or the element name. A passing XSD check alone cannot establish what a downstream consumer will understand.

How to validate and correct the example

  1. Check XML well-formedness. Confirm the document has valid XML syntax, including properly nested and closed elements.
  2. Run the relevant VAST XSD validation. This checks the document against the selected VAST schema. Passing means the structure meets that schema’s rules; it does not test CTV signal semantics.
  3. Identify the intended signal. If the intended signal is `pos`, use the documented `pos` child pattern. If it is `plcmt`, use the documented `plcmt` child pattern.
  4. Validate against the applicable CTV guidance and consumer. Check the final signaling requirements for the relevant format and confirm behavior with the specific player or stitcher where interoperability matters.

Keep these checks distinct: XML well-formedness, generic VAST XSD validity, CTV signaling conformance, and behavior in a particular implementation are separate questions.

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Which version and CTV formats are in scope

IAB Tech Lab identifies VAST 4.3 as released in December 2022. The CTV signaling work is separate guidance, finalized in July 2026 according to the Lab’s CTV ad format signaling overview. Passing a VAST 4.x schema does not mean a document automatically meets that newer guidance, and implementations may differ in their adoption.

The 2026 guidance covers six CTV formats:

  • Pause
  • Menu/Home Screen
  • Overlay
  • Screensaver
  • In Scene
  • Squeezebacks

Five use the OpenRTB Video object with non-linear VAST delivery; Menu follows a Native API path. The relevant path depends on the format, so the presence of VAST alone does not settle whether a particular signaling pattern applies. For VAST background and release context, see IAB Tech Lab’s VAST overview and the VAST 4.3 specification.

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