Expert Advisors is a mechanical trade system (MTS) linked up to a certain plot. The Advisor can not only inform you about a possibility to strike bargains, but also can make deals on the trade account automatically and direct them right to the trade server. Like most trade systems, the MetaTrader 4 terminal supports testing strategies on historical data with displaying on the chart the spots where trades come in and out.

Custom Indicators is an analogue of a technical indicator. In other words, Custom Indicators allow creating technical indicators in addition to those already integrated into MetaTrader 4 terminal. Like built-in indicators, they cannot make deals automatically and are aimed only for implementing analytical functions.

Scripts are programs destined for single execution of some actions. Unlike Expert Advisors, Scripts aren't run tickwise and have no access to indicator functions.

Libraries are user functions libraries where frequently used blocks of user programs are stored.

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