Symptom

Running the procedure "HBEFA-based emission calculation" leads to a warning:
"Emission factors were requested that are not available in the Handbook of Emission Factors (HBEFA). They are listed in the message file."

Cause

HBEFA defines only a subset out of all possible combinations of attributes, whereas in a Visum model any combination might be possible. To avoid the message only use the combinations which are available for the HBEFA calculation.
See also the manual and the document
c:\Users\Public\Documents\PTV Vision\PTV Visum 2024\Examples\PrT HBEFA Emissions\PrT_HBEFA_Emissions_Desc_ENG.pdf
in the section "Analysis of the results".

Workarounds

1) Check the total share of segments which do not have emission factors is below a certain threshold, like 1%.
2) Check whether such segments are typically Euro-0 class, i.e. are likely to disappear or exist in insignificant numbers.