Guest wrote:
.. system say about one trade for instrument..

For any single instrument, you can only have 1 "position" active at any time. A "position" includes pending orders.
You can only widen TP, and can only reduce SL.
SL and TP must be placed within 2% of the price that the trade is, or would be, executed at.
For extra points from Dukascopy in the trading competition, the more orders (entry, take profit, stop loss) that are executed automatically the better. They wish to foster predictive, well thought out, entry based trading styles.
These are a few of the things I have learned in my short time here. If you have other questions let me know, I'll try to help.
ikhall