I don't report this field but this is from page 58 of the FIG:
92 Multifamily Affordable Units Alphanumeric Example: 5 (or) NA (or) Exempt Multifamily Affordable Units
And this is from page 101 of the FIG.
Example: If there are five (5) multifamily affordable units, enter 5.
a. Enter “0” for a covered loan or application related to a multifamily dwelling that does not contain any such income-restricted individual dwelling units.
b. Enter “NA” if the requirement to report multifamily affordable units does not apply to the covered loan or application that your institution is reporting.
c. Enter “Exempt” if, pursuant to the 2018 HMDA Rule, your institution is not reporting Multifamily Affordable Units.
The opinions expressed are mine and they are not to be taken as legal advice.