Ash content
Ash content is the incombustible mineral fraction left after fuel burns, quoted as a percentage by weight. For pellets it defines the fuel grade (under 1% for Premium); for cord wood it is why a stove needs its ash pan emptied every few days in season.
High-ash fuel clinkers up pellet burn pots and buries a wood fire's coal bed, cutting airflow and efficiency. In a pellet appliance, moving from 1% to 3% ash fuel roughly triples the cleaning workload.
A modest bed of ash in a wood stove is actually useful — it insulates the coal bed and helps overnight burns — but beyond an inch or two it crowds the firebox and can hold the door ajar; certified stoves are tested and rated on reasonable maintenance.