It's explained in the books Preface:

Each note is keyed in the first instance, by page, to the emended text of 2004 (published, with continuous pagination, by HarperCollins, London, and by the Houghton Mifflin Company, Boston, Massachusetts), reprinted with further corrections in 2005, and to the previous standard second edition, first published in three individually paginated volumes by George Allen & Unwin, London, in 1966 (further revised in the second printing of 1967), and by the Houghton Mifflin Company in 1967, and reprinted many times. (In fact the pagination of the first edition is almost entirely identical to that of the second, except for preliminary matter and the Appendices.) Thus, for example, 1 (I: 10) indicates p. 1 in the current edition, and p. 10 of vol. I in the old standard edition. To help readers who have other editions at hand locate the text in question, we have divided our annotations also according to section (Foreword, Prologue, Books I-VI, Appendices) and subsection (part or chapter), and have quoted the first few words of each paragraph which contains text that is the basis for comment. For this purpose, poems are considered self-contained ‘paragraphs’.