While the essential dignities are not really critical for discussing or calculating Arabic Parts or Lots, they developed around the same time period and give a sense of the strengths of the planets involved. In particular, there are a very few of the formulae for lots for which you have a choice of planets depending on which one is stronger. It is these dignities and debilities which determine that strength. Roughly, the strength of the planet in question is stronger if it is listed towards the top of the Ptolemaic chart. In order of strength, Rule is strongest, followed by the next strongest: Exaltation, etcetera. The last two rows of the same chart are considered debilities, ie. "bad" or "weak". Naturally, actually using these is somewhat more subtle than I have described here, and I encourage all interested readers to refer to other texts on this subject. I can especially recommend anything by Robert Zoller or Anthony Louis (who gives a brief but effective review of the topic). Lehman's book called "Essential Dignities" is masterful, but a challenge to the reader unless they have extensive astrological experience. The Hindu/Chaldean chart at the bottom of the page refers to a historical view of dignities and is not now used in standard practice for the calculation of strength of the planets. It is provided for reference and comparison purposes only.

Ptolemy's Essential Dignities and Debilities as Used by Lilly

Ptolemy's Azmene Degrees

These are said to indicate lifelong infirmity if conjunct the Ascendent, ruler of the Ascendent, Sun, or Moon.

Hindu Decans & Chaldean Face Rulers

• Hand, Robert. Dignities and Debilities as Collected from a Variety of Sources. http://www.robhand.com/alldign.htm.
• Lehman, Lee.
  • Almutens by Day: http://www.leelehman.com/pages/images/AlmutDay.PDF
  • Almutens by Night: http://www.leelehman.com/pages/images/AlmutNgt.PDF
  • Essential Dignities: http://www.leelehman.com/pages/EssDig.html
  • Glossary: http://www.leelehman.com/pages/glossary.html
  • Triplicities: http://www.leelehman.com/pages/Triplicity.html
  • ... and lots more ... http://www.leelehman.com/
• Lilly, William. "Of the Essentiall Dignities of the Planets." Christian Astrology, CHAPTER XVIII, p. 101+. (1st ed. 1647, reprinted 1985 Regulus). As made available online by Christopher Warnock, Esq. at Renaissance Astrology, URL: http://www.renaissanceastrology.com/essentialdignities.html.