The UEB codebook is an excellent resource for understanding how the rules are to be applied when using the UEB code. There are many instances within Rule 10 (contractions) of when to use and when not to use the contractions. What the code doesn't do is explain "Why Not?".
This series of five Bulletin articles was written to help explain why the contractions can't be used. By referencing other sections of the UEB codebook, this publication clarifies "Why Not".