git sub modules related

Hi

Created a 'super' level module top and add 2 sub modules a and b (As
given in manual); However I am not able to create a Global branch /
Tag.

super ---> a
          |
          ---->b

I am hoping to create a branch FOO which will recursively go in super,
a and b; In that case how do need to associate tags and branch in
super, a and b by a single artifact

Thx/ Mahesh