4.3.1.3 tags reads in part:

*****
This node expresses an array of human-readable tags for the Plan or Specification.
*****

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:

*****
tags: [ PHP, Drupal6, mikez ]
*****

But, an example showing an array, created with comma + whitespace is hardly a normative definition of an array.