I think your members are referring to the volunteer credit or the LOSAP.
The Maryland Income Tax Subtraction Program allows a volunteer who meets certain
requirements to receive a $3,500 credit on their Maryland State income taxes. To qualify for the subtraction modification, a volunteer with less than 25 years of service must have been active in the past calendar year and have at least 36 months of qualifying service in the last ten years. A qualifying member must have a minimum of 50 points and those points must be earned in at least two categories. A member with 25 or more years of qualifying service need not have been active in the previous calendar year; however, the member must still qualify with at least 36 months in the last 10 years.
The National Guard is mentioned in the Length Of Service Award Program. Here is the link to the document that include info and a computation sheet for subtracting tax:
I sincerely XXXXX XXXXX information is helpful and thank you for what you do,