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My understanding is that when you 'invest' in the stock market (long term play, to minimise risk, counteract inflation, etc.), you really do care to get ownership of the actual security you paid for. However, when you 'trade' on the stock market, expecting to sell off the security soon after buying it (likely within the same day), you do not really care to own it as such and that the 'security' might as well be just an entry in a database which grants you the right to 'sell' it again sometime in the future (hopefully getting back more that you initially paid for it).