Well mostly people fill out all the boxes, from 1 to 12. Very few who would want to leave any group. So basically allocate a number to every group in your order of preference.

There are groups that one is highly desirous of joining, but if you are not able to attain that group, will you not want to join any other service group????
I would personally like being a CSP whichever group it is , but yes there are a couple of groups that I am very keen about.

So, fill out all the boxes in your order for preference, and only if you would not want a specific group( i believe alot of girls would not want to be part of Police Service) then dont number it.

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*Whirling Dervaish*