on init happens at the EA start and not again, on timer works every time your setting say it can (default is set to 60 seconds) so every 60, on tick is where you will put the majority of your blocks runs every block every time there is a tick (every price change), on trade works with trades that are running, on chart works with what ever you can visually see on your screen (on mt4 this does nothing on backtest), on denit works once if you remove EA from system
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