Microsoft.Office.Tools

This consists of managed assemblies that provide the communication layer between Office and your solution.
All these assemblies are installed when the VSTO Runtime is installed.
These extensions are specific assemblies that your solution will use in order to automate and extend an Office application.
When you create an Office solution in Visual Studio references to these assemblies will be added automatically depending on the type of solution you are creating.
There are slightly different assemblies depending on which version of the .NET Framework you are using.

Microsoft.Office.Tools namespace

This namespace and assemblies contain a set of types that extend and support a specific object model.
For example in Excel, the Microsoft.Office.Tools.Excel namespace includes the host control objects (ListObject, NamedRange, Chart and XMLMappedRange).
This namespace also includes types that provide smart tag functionality, base class for application-levels add-ins and classes for creating custom task panes in add-ins.
Microsoft.Office.Tools.Excel
Microsoft.Office.Tools.Word

The installation of the Office Extensions for .NET Framework depends on the configuration of the computer and the methods of installation.
Install on Demand - If one of the Office Extensions cannot be installed when the runtime is installed the runtime will automatically install the missing office extension later when the requirements are met.