MQL4 Reference 
Predefined variables
For each executable MQL4 program, a number of predefined variables is supported that reflect the state of the current price chart
at the launching of a program: an expert, a script, or a custom indicator.
Libraries use variables of the module that has called a library.
To have a safe and quick access to these data, client terminal provides local copies of predefined variables for each launched
program separately. These data are updated at every launch of an attached expert or a custom indicator automatically or using the
RefreshRates() function call.
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