In the ASN.1 for ScanResponse, there's an element:

         attributeSet    [8]     IMPLICIT AttributeSetId OPTIONAL

There is no description of this element, neither as an ASN.1 comment, nor is it mentioned in the service definition. What is the role of this attribute set identifier?

It pertains to attribute lists referenced elswhere in the ScanResponse:

• In TermInfo:

  TermInfo ::= SEQUENCE {
     ......
     suggestedAttributes          AttributeList OPTIONAL,
     alternativeTerm        [4]   IMPLICIT SEQUENCE OF AttributesPlusTerm
    .......
  

• in OccurrenceByAttributes

   OccurrenceByAttributes ::= SEQUENCE OF
  SEQUENCE{
        attributes      [1]    AttributeList, 
  .......
  

In these cases, where AttributeList or AttributePlusTerm are referenced, both reference attributeElement, which may or may not include an attribute set identifier; attributeSet specifies the attribute set identifier in cases where it is omitted in attributeElement.