Starts with double slash

A leading // on a pattern selects nothing extra. Every XSLT pattern is matched unanchored — a node matches when any alternative of the pattern matches it, wherever it sits — so match="//item" and match="item" select exactly the same nodes, every item at any depth. Read carelessly, the // suggests a scan of the whole document that never happens.

What it does change is which rule wins. A pattern's default priority comes from its shape: a lone name test scores 0, while a pattern carrying a / step scores 0.5. So //item outranks item by half a point, and two templates that look interchangeable are not:

<xsl:template match="list/item">SPECIFIC</xsl:template>
<xsl:template match="//item">DOUBLESLASH</xsl:template>

Both patterns score 0.5, so the later one wins and every item inside a list is handled by DOUBLESLASH. Remove the // and list/item keeps 0.5 while the bare item drops to 0, so the same node is handled by SPECIFIC instead. The output changes though neither template was touched.

An alternative of a union is matched exactly as a whole pattern is — the node matches when any one of them matches it — so the same // is as redundant in match="chapter | //item" as it is alone, and it buys the same half point: XSLT computes a default priority for each alternative separately, as though the rule had been written out once per branch. Write each branch the shape you mean:

<xsl:template match="chapter | item">CONTENT</xsl:template>

Where a rule has to keep the rank it had, say so with an explicit priority rather than leaning on a // to buy half a point:

Incorrect:

<xsl:template match="//item">
  <xsl:value-of select="."/>
</xsl:template>

Correct:

<xsl:template match="item" priority="0.5">
  <xsl:value-of select="."/>
</xsl:template>

The redundancy is the same in every attribute that holds a pattern rather than an expression — xsl:key/@match, xsl:accumulator-rule/@match, the @count and @from of xsl:number, and the @group-starting-with and @group-ending-with of xsl:for-each-group. None of those is anchored either. Priority is a template's concern alone, so in the others a leading // costs only the misreading.