monobloc tap with water carbon filter?

[sasik]

Hi

Did any of you fitted a monobloc kitchen tap with a carbon filter? I was in one of my customers house this morning to re-fix a lose monobloc tap back onto the kitchen sink, but when I looked under the tap I could see a thin tube coming from a filter into a long brass screw. The brass screw is hollowed and it's used to secure the body of the tap to the sink and it's the inlet of treated water from the filter. I don't understand how this thing work, because this tap have normal cold inlet straight from the main, hot water inlet from the combi and inlet of cold from the filter😕 So how dose it work?

Thanks Sasi