Specifies source trees for files.
| Absolute | The file path is absolute path. |
| Source | The file path is relative to the project path. |
| Group | The file path is relative to the group the file is in. |
| Build | The file path is relative to build products path. |
| Developer | The file path is relative to the developer directory. |
| Sdk | The file path is relative to SDK path. |