Along with expert advisors and custom indicators, it is also possible to create and use scripts and libraries. The main difference between a script and an expert advisor is the way they are used: Expert Advisors are executed for each tick, while Scripts are executed only once, on request. For example, with a script, you can implement various sequences of actions for the client terminal, displaying additional messages or waiting for some actions from the user.

Libraries are used to create archives with custom functions. It means that a software developer can collect her/his original functions in library files and call them from any custom program. As soon as a library is created all functions described in it are compiled and saved for the future use. It is worth mentioning that functions being not called are removed from the code of expert advisors, custom indicators, and scripts during the compilation process in order to optimize it. This is the peculiarity of creating libraries in MQL 4 as compared to other programs.