What does Buy/Long Trailing Stop mean in futures? And why can I only set price below the current market price? 
Does it mean the Long Position order will be triggered when the price goes down until it reaches the set price, and then the Tailing Order will be activated? After that, when the price increases until it decreases(for 2% B/C), the Trailing order will be closed and I get profit from the increase in price? (since it was a long position)

Also, from what I understand, I should
first open a Long/Short position
and then set/open the Trailing stop order for that order (so the amount or the two orders should be the same)
What I'm confused is the opening trailing stop. Does it mean I don't need to have an already opened position and the trailing stop will start right at the Trailing Stop order?
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