Hello,

I am not an authority on form completion or its queries but I believe you need to mention all those groups for which you wish to be considered, If your First choice is Foreign Service then Write 01 against it, if your Second choice is DMG write 02 and if you do not wish to be considered for Income Tax then left it blank.
I hope it will help.
It is my personal undertstanding, if wrong please correct me.

Thanks.