for chemical equilibrium, what is the difference between percent ionization and percent dissociation
None. Many people, and I include myself, misuse the two terms because we often use them interchangeably. Actually, they are not.
Two examples.
1. NaCl is a solid crystal that consists of Na^+ and Cl^- bound together in a crystal lattice. When this crystal is placed in water it dissolves and DISSOCIATES into the hydrated ions. Most of us say it ionized but actually it did not. Ions were already there; the ions already there dissociated from each other to from the hydrated ions in solution. Acetic acid, which has no ions in it (covalent bonds and pure acetic acid does not conduct electricity) reacts with water to form ions.
CH3COOH + H2O ==> H3O^+ +_ CH3COO^- and we should say it ionized in solution because we took a material with NO ions and made ions from the molecule.