PLEASE... dont quote the book.. thats for exam purposes, and was devised to create standardisation, and much of it does not relate with reality ,into todays interpretations of the practical application of advanced theories.
Remember, that was written in 1948.... there have been many changes in techn., music and style since that time ( I know, Ive lived , danced and taught thru them ) .
There is an ongoing discussion in great detail on a far more technical level, on this very subject , on Dance Forums .
There are 3/4 dominant styles currently being proffered to the comp. arena...... Italian, Classic English and Round , which many coaches are teaching, with no consensus . In addition to those, add the " body " school also prominent.
Let me pass on this nugget for you to consider... world famous coach once stated ( para phrased )
" no technique can be firm, and any may change due to circumstance ".