Those who copied the Canterbury Tales after Chaucer's death did not always agree about how the tales should be arranged, and a moment's reflection will show why this might have been so.
The General Prologue (GP) obviously comes first. The Parson's Prologue and Tale (ParsT) just as obviously come last. How can we know how to arrange the tales in between?
At the end of the General Prologue, the narrator describes how "the cut fell to the knight" (GP 845), thereby linking the GP to the Knight's Tale. Another link at the end of the Knight's Tale describes how the Miller breaks in to tell his story, and after that the Reeve, and then the Cook. But the Cook's tale breaks off, incomplete, with no indication in the text as to what comes next.
GP, KnT, MilT, RvT, and CkT are thus linked together to make up Fragment I. Similarly some other tales are clearly linked to one another by connecting text, while a few stand on their own with no explicit links to any other tale.
Editors of the tales must therefore decide how to assemble these fragments into a whole. The arrangement given above, using the Roman numerals from I to X, is the most usual (though not the only) one.
For fuller discussion with suggested readings, see Larry D. Benson, The Riverside Chaucer, 3rd ed. (Boston 1987), 4-22 and 797-97.