The first number is the "new" decimal-style checklist number, corresponding to "A checklist of the Lepidoptera of the British Isles" by Agassiz, D.J.L., Beavan, S.D. & Heckford R.J. 2013, hitherto referred to as the "2013 checklist".
The second number, prefixed with "BF" is the previous "Bradley and Fletcher number", derived from "A Recorder's Log Book or Label List of British Butterflies and Moths" by J.D. Bradley and D.S. Fletcher.
Some species will have "Appendix A" to the right of their listing. This relates to Appendix A of the 2013 checklist, which lists adventive species.
At the end of the main list are Questionable or Doubtful records which is an approximation of Appendix B (Questionable records) of the new checklist, but also contains other records that do not for various reasons have new decimal-style checklist numbers.
Many thanks to Stella Beavan and Bob Heckford for their help in interpreting the new checklist.
This section lists those species recorded whose records are considered doubtful or questionable (Appendix B in the new 2013 checklist).
In this section you will also find some misidentifications, reclassifications and other records that haven't yet been properly placed (by me). I'm still working on that!