Dimensions spec - problem in 2.5?

The explanations beginning section 2.5 (see below) from the dimensions
spec. is confusing.

First sentence (see below)  talks about "xbrldt:dimensionItem
substitution group", and the last sentence  talks about "xbrli:balance,
xbrli:periodType and nillable attributes" ... none of which are shown in
the xml schema block between the two sentences.

Problem?

regards,

bill p

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2.5 Dimensions (normative)
[Def, 5] A dimension declaration is an abstract item declaration in the
xbrldt:dimensionItem substitution group.  A dimension is each of the
different aspects by which a fact may be characterised.
  <xs:element
    name="dimensionItem"
    id="xbrldt_dimensionItem"
    abstract="true"
    substitutionGroup="xbrli:item"
    type="xbrli:stringItemType"
    xbrli:periodType="duration"/>
The xbrli:balance, xbrli:periodType and nillable attributes of a
dimension item declaration have no significance.