Every attribute name carries a kind, which convert_scale() consults:
SCALE_ATTRIBUTE_KINDSA character vector of length 2.
A character vector of the two attribute kinds accepted in the
kind column of attach_attributes_scale(): "property" and
"factor".
propertyA physical property of the member, an invertible ratio: a calorific value in GJ/t converts tonnes to gigajoules and gigajoules back to tonnes; a density, a yield.
factorA one-way multiplier: an emission factor in kg/TJ turns energy into CO2, and there is no physical sense in which CO2 turns back into energy; a loss factor, a tax rate.
SCALE_ATTRIBUTE_KINDS
#> [1] "property" "factor"