By the class of the substance - tannin, protein, oil, dye or general soil - because each class responds to different chemistry and is set by the wrong one.

Tannin stains from tea, coffee, wine and fruit need an acidic tannin remover. An alkaline general cleaner will set them, and once set they are very difficult to remove.

Protein stains from blood, milk, egg and body fluids need enzyme chemistry and cool water. Hot water cooks protein into the fibre permanently.

Oil and grease need a solvent-based product; water-based cleaners cannot dissolve them.

Dye stains - felt pen, food colouring, some medicines - need reducing or oxidising chemistry that risks removing carpet dye as well, and should be approached carefully and tested first.

General food and drink spills often respond to a neutral detergent spotter, which is the right first attempt when the substance is unknown.

Rubbing distorts the pile permanently and drives the substance deeper into the fibre and the backing.

Carpet fibre has a twist that gives it appearance retention. Vigorous rubbing untwists and frays it, producing a permanently dull, fuzzy patch that remains visible after the stain has gone.

Rubbing also pushes liquid down into the backing where extraction cannot easily reach it, which is exactly what causes a spot to wick back later.

Blotting with a clean white absorbent cloth transfers the substance out of the carpet and onto the cloth. White matters - coloured cloths transfer their own dye.

Work from the outer edge towards the centre so the spot is not spread outwards, and keep turning to a clean part of the cloth as it picks up colour.

Wicking - liquid left in the backing rises to the surface as the carpet dries and brings the residue with it.

Most spills penetrate below the visible pile. Cleaning the surface removes what can be seen while leaving material in the backing and lower pile.

As the carpet dries, capillary action draws that moisture upward. It evaporates at the tips and deposits everything it was carrying there, so the spot reappears - often larger than the original.

Prevention is straightforward: extract as much liquid as possible, then leave a pad of absorbent white towelling weighted on the spot as it dries, so moisture wicks into the pad instead of to the pile tips.

Where a spot has already wicked back, repeat the treatment and use the pad - a second attempt with the same method and no pad usually produces the same result.

No - the strongest product is the most likely to set the stain, remove the carpet's colour or damage the fibre.

Escalating strength is the classic sequence of events behind a permanent stain: a tannin stain treated with an alkaline cleaner, then bleach, then a solvent, leaving a bleached patch where a coffee mark used to be.

The correct escalation is by class, not by strength. Try the mildest appropriate chemistry, give it dwell time, agitate gently and rinse. Repeat before moving to something more aggressive.

Oxidising and reducing spotters are effective on dye stains and are also capable of removing carpet dye. They must be tested in an inconspicuous place and used sparingly.

Where the spot resists correct chemistry applied properly, it is usually a dye stain, a chemical bleach mark, or fibre damage - none of which is fixed by a stronger cleaner.

A small set covering the classes - neutral spotter, tannin, protein enzyme, solvent, and a rust or specialist product - plus white cloths, a spatula and a spray bottle.

A handful of products covers almost every spill in a commercial building, and having them on the trolley is what makes prompt treatment possible.

White cotton or microfibre cloths are essential; coloured or printed cloths transfer dye onto wet carpet.

A blunt spatula or bone scraper removes solids before any liquid is applied, which prevents spreading.

Label every bottle with contents and hazard information, as with any decanted chemical.

A written guide on the trolley matching substance to product is worth more than training alone, because spots are treated by whoever is nearest and the decision is made in seconds.

Keep it stocked. A spotting kit missing the enzyme product is why a blood spill becomes a permanent mark.

Rehydrate first, then treat by class with dwell time - and accept that some are permanent because the fibre or dye has changed.

A dried stain has to be softened before chemistry can work. Applying the appropriate spotter and allowing genuine dwell, or pre-wetting with warm water and covering to prevent drying, does that.

Multiple mild treatments outperform one aggressive one on old stains, and each cycle should finish with rinsing and blotting.

Some marks are not stains at all. Fibre that has been bleached, burned, abraded or chemically damaged has no colour to remove, and no cleaning product restores it - the answer is a repair or a patch.

Long-standing tannin, dye and rust marks may be partially reduced rather than removed, and setting that expectation before starting avoids a dissatisfied client.

For valuable or irreplaceable carpet and rugs, specialist treatment is the correct referral rather than repeated attempts.

They can, and it is why a rinse and blot step closes the process rather than being an optional extra.

Any surfactant left in the pile attracts soil, so an untreated residue produces a dark patch exactly where the spot was - which looks like the stain returning but is a different problem.

After the spotter has done its work, rinsing with clean water and blotting removes both the released soil and the chemistry that released it.

Some professional spotters are formulated to dry to a non-tacky residue, which reduces but does not eliminate the issue.

Over-application is the main cause. Applying spotter to the cloth, or misting lightly, puts far less product into the carpet than saturating the area.

Where several spots have been treated in one area over time, an extraction clean of the whole area evens out the appearance and removes accumulated residue.