The only known references to the Law of Planetary Affinity by Alice A. Bailey (AAB) in her voluminous writings and those given to her by Djwhal Khul (DK). Links on these pages of the Laws of Alice A. Bailey are to the online books at the Lucis Trust website.
What is a law?
… a law is but the spiritual impulse, incentive and life manifestation of that Being in which he lives and moves. A Treatise on White Magic , INTRODUCTORY REMARKS
A law is an expression or manifestation of force applied, under the power of thought, by a thinker or group of thinkers. Discipleship in the New Age, Vol. I , SECTION ONE – DISCIPLESHIP IN THE NEW AGE – Part 2
… a law is in reality the effect of the life of a greater entity as it encloses a lesser within its living processes.
Esoteric Healing , Part Three – The Fundamental Laws of Healing
… a law is but the effect of the continued intelligent activity of the Life aspect as it works in conjunction with matter. Esoteric Psychology – Volume I , 5. The Rays in Cyclic Manifestation
A law is only an expression or manifestation of force, applied under the power of thought by a thinker or group of thinkers. Esoteric Psychology – Volume II , II. The Seven Laws of Soul or Group Life – Part 5
A law presupposes a superior being who, gifted with purpose, and aided by intelligence, is so coordinating his forces that a plan is being sequentially and steadily matured. A Treatise on White Magic
Quotes I found below:
9. Law of Planetary Affinity.
9. The Law of Planetary Affinity. This term is used in the occult teaching specifically in connection with the interaction of the planets with each other and their eventual marriage. As we know, the planetary schemes (the seven sacred planets) will eventually synthesise, or absorb the life of the planets which are not termed sacred and the numerous planetoids, as far as the four kingdoms  of nature are concerned. The absorption of the Spirit aspect proceeds under the Law of Synthesis. The minor four planetary schemes become first the two, and then the one. This one, with the major three, forms a second and higher quaternary which again repeats the process, producing from the four, the two, and from the two, the one. This final one is eventually merged in the Sun, producing in this prolonged process, and over a vast period of time, the appearance of the “seven Suns who run together, and thus blaze forth, producing one flaming ball of fire.”
You are welcome to post your comments and understanding of this law.