The Short Answer - They tell us both what’s inside your catalytic converter or DPF

MORE INFO

Instead of the manufacturers making a different casing every time they changed the value of the contents, it made sense to imprint a PPM code

We can enter that PPM code into the database to much more accurately appraise the material using previously taken test samples of that code

Without checking that code, it’s up to chance what you actually have, which means you’re missing out on all of your special high value codes

And, your buyer is probably being rewarded on average for not putting the effort in to find those and pay you as accurately as possible

Suppliers rely on their buyers for information about their material and they’ve been historically let-down on that front, In a sort of wild-west format roping them in with great service at first -

Then punishing them for their trust longterm.

More Informative Answer

They’re the catalogued, readily available conversation between the previously taken laboratory tests, and the buyer - Working together to have on the spot knowledge when paying suppliers as accurately as possible

Most catalytic converters and DPF systems will have this imprinted, most commonly being on either the outer metal casing, on the heat shield etc.

Less commonly they’ll be directly imprinted into the exhaust pipe below like for some Chrysler manifolds - On brackets further down the exhaust on makes like Subaru - On the Exhaust Manifold

And So On

Imprinting a code like this during manufacturing is among the most cost effective ways to tell the later recyclers what PGM recipe was used in its creation

But it’s not the only way.

Honda especially has been known to use other forms of communication, like the O2 sensor positioning and other physical features of the outer casing of the material

For a proper appraisal, it’s important to find, record & use as these many of these codes and specifications as possible (generally one per unit)

The market hasn’t historically been able to depend on its buyers to leverage this information to offer the best possible service

So it’s coming recommended that the people invite as much education as possible to lessen their dependence on information given by buyers