In my reading today, I came across more information about the Explanatory Notes:
During the previous decade, John had hoped the publication of his Bible commentary, Explanatory Notes Upon the New Testament, would provide doctrinal help for his preachers. The first edition, in 1755, had been prepared more hastily than Wesley had hoped. The second edition the following year was essentially a reprint, though with the errata incorporated. In 1760, however, he and Charles had embarked on a major revision of the work, further refining the biblical text and expanding the notes. They finished this new edition in 1762 and, combined with the collected Sermons on Several Occasions John had published (four volumes by 1760), it provided basic doctrinal guidelines for the preachers.
By the late summer of 1763, Wesley had firmly fixed these two resources as the measure of proper Methodist preaching. (Heitzenrater, Wesley and the People Called Methodists, 212-3)
Heitzenrater goes on to argue that the Model Deed, which controlled access to Methodist pulpits, stipulated that preachers must preach “‘no other doctrine than is contained in Mr. Wesley’s Notes Upon the New Testament, and four volumes of Sermons.’ By this stipulation, the Sermons and Notes became the doctrinal standards for the Methodist preachers.” (Heitzenrater 213)
If you are still reading, you will see that we are starting to get somewhere… The Explanatory Notes were part of the doctrinal standards of early Methodism because they were considered to be an important way of ensuring that the people who preached in Methodist pulpits were preaching a doctrine that Wesley would approve of. Thus, the Explanatory Notes were intended to play an important role in defining what was acceptable Methodist teaching.
This still leaves open for discussion the role that they do actually play today and the role that they should play today.