i am giving exams in IDTA i only know istd is there a diference in american style ?
which is better
my instructor tells me that in american style latin it is better than istd !
can someone tell me ?

Better ?-- moot point-

it depends very much on your location. If you are in the States, there is far more opportunity, to dance what you learn , with a greater number of people.

The english equivalent, has many similarities , and again, depending on whether you choose social and or the more competitive style , could affect your usage ( if no partner )

They, both styles, may be danced in competition . Again-- decide the path, you wish to persue .
The bronze american style rhythm, has more dance variety from which to choose .
Mambo-- salsa --rumba-- bolero-- cha cha-- samba-- west coast swing -- east coast swing-- merengue-- hustle .
The availibility , of a teacher close at hand, might determine your choice .

As far as actual differences are concerned , too many to list here . essentially, the concepts are the same. The approach may differ in certain dances .

I don't know anything about the IDTA, so I did a little surfing to try and learn something. I'm still not sure I fully understand what it is. Information I was able to find was not very specific. It appears to be an umbrella organization for the promotion of many different dance forms, but primarily teaching oriented. There seems to be a heavy emphasis on teaching kids. Membership to this organization is by examination. The organization is worldwide, but by far most members are located in the UK.

IDTA dance styles listed were:
Salsa, Mambo, Merengue, Samba, Rumba, Cha Cha, Argentine Tango, Tango Ballroom, Paso Doble, Waltz, Viennese Waltz, Foxtrot, Quickstep, Freestyle/Disco, Streetdance, Theater Craft, Musical Theatre, Modern Jazz, Jive, Tap, Ballet, Classical Sequence dance, and Rock ‘n' Roll.

Notable missing from that impressive list are some of the uniquely American styles such as bolero, hustle, east coast swing, west coast swing, and Lindy hop. From this I am thinking that the organization is primarily concerned with international style dances. Is this correct?