4.3.1.3 tags reads in part:
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This node expresses an array of human-readable tags for the Plan or Specification.
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However, its data type is listed as string[ ]. Moreover, none of the other 51 instances of "tags" defines the representation of this "array."
Actually that's not entirely true because Example 7 (non-normative Example 7) does show:
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tags: [ PHP, Drupal6, mikez ]
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But, an example showing an array, created with comma + whitespace is hardly a normative definition of an array.