The CRI first focused the level in which cleaning products re-soiled faster than the same carpet not cleaned. The industry responded by adding polymer (residue) that neutralized the re-soiling impact of certain surfactants or solvents like d'limonene. Encapsulation cleaning is based on leaving residue that limits re-soiling. The use of some ingredients like phosphates reduce the level of residue left on the carpet. In short, everything leaves residue!! The question is how much and will it cause rapid re-soiling or a health hazard.
Protector leaves fluorochemical and acid dye resistor residues. You can compare the quality of any protector product by determining the level of solids. The more solids, the better performance. The limit to the level of solids is whether it changes the texture of the fiber. A stiff fiber has too much protector.