The import construct is the re-usability mechanism in msbuild. When an import element is encountered, these steps take place
The working directory is changed to that of the imported project file.
Project element attributes are processed.
If a value is already assigned to DefaultTargets attribute, then this is ignored, otherwise it becomes the value of DefaultTargets. If InitialTargets attribute is present, then the list of targets will be appended to the current list of InitialTargets.
Project element nodes are processed.
The working directory returns to importing the previous value.