REP Weave - Warp Colour Sequence - Layers

I recently purchased the REP, RIPS, REPS Weave book by Lucienne Coifman.  In the section on ‘How to Read Drafts’ there is a paragraph on Warp Colour Sequence.  The diagram is simple to follow however I’m a little confused by the use of ‘Top Layer’ and ‘Bottom Layer’. I have no problems understanding the Threading Draft which appears the same as any other draft. Can anyone explain what is meant by Layers and why the term is used as I can’t see the relevance when it comes to threading?

Thank you.