It is a
US legal restriction related only to acquisitions of
US equities and single stock futures/
You can trade up to 4 trades in a 5 day period before being classified as a pattern day trader, although some brokers will draw the limit at 3 to stop falling foul of the requirement.
The link below gives a detailed explanation from one US broker
http://www.interactivebrokers.com/en/p.php?f=margin&ib_entity=llc
Charlton