When describing a GeneXus application, the developer does not have to consider, for example, what programming language it will be implemented in or what DBMS will be used. Such "details" must be defined, for example, at the time of testing the application. Each set of implementation values is known as Environment. The most common use is for DBMS/Programming language combinations. You will usually see Environment names like "Java DB2 UDB", "Java MySQL", "C# SQL Server", etc.
For further details, see: Environments