From publisher blurb:
Compendium Arcanum Volume 9: 8th-Level Spells continues where volume 8 left off! This book includes 55 pages of 8th-Level spells for the Pathfinder Roleplaying Game, now with Heightened and Diminished effects!
A multi-variable spell is a spell which, once learned, you can cast as a spell one level higher or lower than the original spell. This mechanic was first introduced in Monte Cook’s Arcana Evolved. Multi-variable spells give spellcasters great flexibility and so Tim decided to begin by adding the Heightened and Diminished qualities to Pathfinder spells.
So, in addition to the normal Pathfinder spell details, you will see one or two new bits of text, specifically, either a “Diminished Effects” line, a “Heightened Effects” line, or both. This is the first book of a set intended to include all spells from the Pathfinder Roleplaying Game Core Rulebook, Advanced Player’s Guide, Ultimate Combat, and Ultimate Magic, now with heightened and diminished effects.
This section describes changes to the spell if it is cast using a slot one Level lower than normal. You must be able to cast a spell at its regular Level before you can use the diminished effects. If you use a diminished effect, treat the spell as one Level lower than listed for purposes of spell slot used, saving throw DCs, magic item pricing, and so on. Spells that are 0-level (or 1st-level alchemist-, paladin-, or ranger-only spells) have no diminished effects.
This section describes changes to the spell if it is cast using a slot one Level higher than normal. If you use a heightened effect, treat the spell as one Level higher than listed, for purposes of spell slot used, saving throw DCs, magic item pricing, and so on, regardless of what Level slot you use. Spells that are 9th Level (or 6th-level alchemist-only spells or 4th-level paladin- or ranger-only spells) have no heightened effects.