Section https://www.w3.org/TR/xslt-30/#merge-examples has an example with code

  <record ID="{$merge-key}">

although that variable is not defined in the example. I suppose the example has not been (completely) corrected since earlier versions of the spec where merge groups and keys were accessible as variables.

In the current spec I think 

  <record ID="{$merge-key}">

should be

  <record ID="{current-merge-key()}">


And in https://www.w3.org/TR/xslt-30/#merge-action there is an example using an undefined variable 
  
  <xsl:copy-of select="$master"/>

which should be

  <xsl:copy-of select="current-merge-group('master')"/>

to use the proper way to access a merge group by name in line with the current spec.