The Classicthesis LyX template presets the Appendix as a chapter (or a child document), with chapter number "A". So the first section appears as A.1, and the first subsection appears as A.1.1 and so on in the Classicthesis file (or the parent document). That is, all subsequent sections and subsections are numbered starting with "A".
I want to customize the Appendix Chapter in Classicthesis LyX version 4.2 to have
A Appendix for Chapter 1
B Appendix for Chapter 2
C Appendix for Chapter 3
This style is commonly used in PhD thesis, which is perfect for many equations, figures and tables in Appendix. For example, the fifth equation in Section C is labeled as (C.5).