When generating diagnostic messages, PVS-Studio by default generates absolute, or full, paths to the files where errors have been found. That's why, when saving the report, it's these full paths that get into the resulting file (XML plog file). It may cause some troubles in the future - for example when you need to handle this log file on a different computer. As you know, paths to source files may be different on two computers. This will lead to you being unable to open files and use the integrated mechanism of code navigation in such a log file.
Although this problem can be solved by editing the paths in the XML report manually, it's much more convenient to get the analyzer to generate messages with relative paths right away, i.e. paths specified in relation to some fixed directory (for example, the root directory of the project source files' tree). This way of path generation will allow you to get a log file with correct paths on any other computer - you will only need to change the root in relation to which all the paths in the PVS-Studio log file are expanded. The setting 'SourceTreeRoot' found on the page "PVS-Studio -> Options -> Specific Analyzer Settings" serves to tell PVS-Studio to automatically generate relative paths as described and replace their root with the new one.
Let's have a look at an example of how this mechanism is used. The 'SourceTreeRoot' option's field is empty by default, and the analyzer always generates full paths in its diagnostic messages. Assume that the project being checked is located in the "C:\MyProjects\Project1" directory. We can take the path "C:\MyProjects\" as the root of the project source files' tree and add it into the field 'SourceTreeRoot', and start analysis after that.
Now that analysis is over, PVS-Studio will automatically replace the root directory we've defined with a special marker. It means that in a message for the file "C:\MyProjects\Project1\main.cpp", the path to this file will be defined as "|?|Project1\main.cpp". Messages for the files outside the specified root directory won't be affected. That is, a message for the file "C:\MyCommonLib\lib1.cpp" will contain the absolute path to this file.
In the future, when handling this log file in the IDE PVS-Studio plugin, the marker |?| will be automatically replaced with the value specified in the 'SourceTreeRoot' setting's field - for instance, when using the False Alarm function or message navigation. If you need to handle this log file on another computer, you'll just need to define a new path to the root of the source files' tree (for example, "C:\Users\User\Projects\") in the IDE plugin's settings. The plugin will correctly expand the full paths in automated mode.
This option can also be used in the Independent mode of the analyzer, when it is integrated directly into a build system (make, msbuild, and so on). It will allow you to separate the process of full analysis of source files and further investigation of analysis results, which might be especially helpful when working on a large project. For example, you can perform a one-time complete check of the whole project on the build server, while analysis results will be studied by several developers on their local computers.
You can also use the setting 'UseSolutionDirAsSourceTreeRoot' described on the same page. This setting enables or disables the mode of using the path to the folder, containing the solution file *.sln as a parameter 'SourceTreeRoot'. When this mode is enabled (True), the field 'SourceTreeRoot' will display the value '<Using solution path>'. The actual value of the parameter 'SourceTreeRoot' saved in the settings file does not change. When the setting 'UseSolutionDirAsSourceTreeRoot' is disabled (False), this value (if it was previously set) will be displayed in the field 'SourceTreeRoot' again. Thus, the setting 'UseSolutionDirAsSourceTreeRoot' just changes the mechanism of generating the path to the file, allowing to use 'SourceTreeRoot' as a parameter or a specified value or a path to a folder, containing the solution file.