The Maryland instructions specifically state that if you have only one code that applies to form 502SU that you do not need to add this form to the return. "If only one of these subtractions applies to you, enter the amount and the code letter on line 13 of Form 502; then the use of Form 502SU may be optional." (page 8 at Maryland Individual 2019 Tax Booklet)
[Edited 3/26/2020 10:02 am CDT - updated for 2019]
