ConcurTaskTrees (CTT) is a notation for task model specifications useful to support design of interactive applications specifically tailored for user interface model-based design.
The main features of ConcurTaskTrees are:
In human–computer interaction, task models indicate the logical activities that an application should support to reach users’ goals. There are also methods that indicate how to derive user interfaces for different platforms from ConcurTaskTrees specifications The last evolution has been the introduction of preconditions.
It has been used both in academia and industry, especially by companies with an interest in enterprise resource planning (ERP) and safety-critical systems (e.g. air traffic control systems). . It has been considered in the World Wide Web Consortium (W3C) for standardization of task models .
Some studies have investigated its usability 
ConcurTaskTrees has been mapped into Unified Modeling Language (UML).
Tool support.
The editing and analysis of task models is supported by the ConcurTaskTrees Environment (CTTE).
The executable code is publicly available and free to download. 
References.
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