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Property collectors determine how the server or agent finds a tool. The default property collectors describe the locations where the agent might look to find a tool, so for example the NUnit 2.4.8 property collector tells the agent to look for a file named nunit-console.exe in either %PROGRAMFILES%\NUnit 2.4.8\bin\ or %PROGRAMFILES(x86)%\NUnit 2.4.8\bin\ or on the %PATH%. If it finds the file there it will report a property nunit.2.4.8.path, if doesn't find the file then it will not have that property.
You can override the default property collectors, or add new ones. Property values can also be manually overridden on an agent by agent basis.
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