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The "Subjects" tab allow to enable\disable engines based on supported "subjects".

By default all engines registered in "Common" group so additional tune required only when needed. By enabling some subject the special "tuned" versions of services which support some subject will be added to list of accessible services.

The best scenario of use "Subjects" if you has enabled, by example, "Common" and "Informatics" then services which support "Informatics" subject will return two results - one in "Common" subject and other with "Informatics" tuning.

On other hand, if you enable only "Aviation" then only engines with support of "Aviation" will be available, exists drawback of this scenario - you will be very limited in engines\language directions count. So please don't disable "Common" subject if you are not sure.