Short Answer:
They tell you what's in the Catalytic Converter.
T15
KV
KT6045
B5C
Informative Answer:
PPM codes, or "Parts Per Million" codes have attached the concentration of materials within the Ceramic substrate of your emissions system/s. They and sometimes other specifications are required to pull previous test samples, in order to compensate customers according to the most accurate data available.
PPM codes were arguably generated as a cost-reduction method during manufacturing while editing the platinum group metal "recipe" for new models.
Instead of designing new casings for every recipe, a "PPM code" could be stamped which would describe its contents to those later recycling.
Rough Example
Code "772AA" May Contain:
(A) X grams of Platinum (Pt)
(B) X grams of Palladium (Pd)
(C) X grams of Rhodium (Rh)
Value = "Sum" of A/B/C individually multiplied by
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