When talking about modems, the UK uses Annex A (xDSL over POTS) and some other countries such as Germany use Annex B (xDSL over IDSN).
Please understand that annex simply means appendix. Technical documents often have appendices, which we refer to as Appendix 1, etc.. In continental Europe, in the English translations of technical documents, these appendices are referred to as Annex A, etc.. I believe this was the result of a piece of bad translation years ago which became standard practice.
So the Annex A which we're talking about here refers to an appendix to a certain technical document which defines part of the internet infrastucture. The Annex B referred to by Asbokid and others refers to an appendix to a different technical document which defines types of xDSL.
Modems which describe themselves as Annex A or Annex B are talking about the infrastucture documents.
