What does demineralization cartridge mean in practice?
Mixed resins remove remaining cations and anions, so the cartridge works most efficiently with water pre-purified by RO.
When does it matter?
When connected to tap water it will run out quickly and the onset of breakthrough requires a low conductivity measurement, not a simple taste test.
Which solutions are related to it?
It is used as a DI stage, controls the conductivity downstream of the cartridge and replaces the media before the receiver exceeds the limit.