Rage is a defined term for spells. While I don't think the golossary has a definition of "fixed" I've usually figured that for game purposes it means "doesn't scale with level" in that particular context (e.g., Arcane Mark has Range: 0 ft, as do Clone, Flame Blade, Ironwood, Minor Creation, Phantom Steed, Phase Door, Produce Flame, Simulacrum, Summon Instrument, the Symbols (sort of), and Teleportation Circle)