first of all, welcome!
and glad u haf a positive attitude towards ur ns too.
i frm inf. so wat i can say here is frm my limited knowledge.
there r 2 types of bmt - mono intake or bmtc.
mono-intake - u go to ur unit for bmt. hence u dun haf a choice
wich vocation u land in. it mabbe gds, inf, cdo... watever. but
once u r there u r there for ur entire ns. unless o cos if
accidents happen, etc.
for bmtc, i guess u can state ur interest, but im not sure how much
tt wil influence ur posting after bmt.
if u kanna posted to gds, cdo frm bmtc, u wil go there and go 4
their conversion cses, details of wich u haf to find out from
others here. after tt u r one of them.
or u may go to advanced schs like sispec and ocs.
frm there, o cos u may get posted to gds or cdo, etc after ur cse
too. and similarly u go 4 their conversion cses den.
as for ppl signing on, u wil go bmtc for ur bmt, and den either go
to the unit, go sispec or ocs, depending on the appointment u sign
on to. after tt den u get posted to one of the units for tt
vocation and go 4 conversion.
hope tt helps. 
