When a statute or statutory instrument is first passed its sections are numbered sequentially: s1, s2, s3, s4 and so on.
Over time, two things tend to happen. Sections are repealed (a lawyerly word for deleted). This is fairly easy, the section simply drops away and is forgotten.
Second, new sections are inserted. This is done by adding a capital letter to the number. So if a new section is inserted between s14 and s15, it is s14A. If there is more than one section to be inserted, you progress through the alphabet: s14A, s14B, s14C on so on. Inserted sections always use capital letters to avoid confusion with the lower case letters used in subparagraphs, eg s99(1)(a).
Occasionally, its necessary to insert sections between two sections that were themselves inserted. For example, to insert a new section between s14A and s14B. Then we add more letters. So the new section would be s14AA.
Of course, in the context of a tribunal case, you won’t be amending legislation. But it’s useful to understand how this works, because it makes it easier to understand the history of a piece of legislation. It’s also important to realise that inserted sections are wholly freestanding. So, for example, a cross reference in legislation to s101 does not apply to s101A.