HTML customizations

Hi dev,
      I am going through the book "DocBook XSL: The Complete Guide - 4th Edition" and in chapter 12 book addresses  class.value mode and the class.attribute mode.I got the facts regarding the class.value mode but have the below doubts regarding the latter.

    How can we indicate the customisation layer about the class.value?

    What is the relationship between the class.value and class.attribute mode?       

 Any clarification will be a great help for me.

Thank you.
Sanjaya Liyanage.