The main model uses Universally Unique ID (UUID) to connect with the harnesses. The UUID information can be found in harnessinfo.xml file. So the directory information is not included in the harnessinfo.xml file.
If the harness is saved outside the default working directory, you can use the function to include the harness directory to MATLAB path:
addpath('the directory of the harnesses')
Then when the main model is opened, the harness can be recognized automatically.
If the harness is from a different model, the harness has to be imported to the main model to be recognized as its harness by the main model. Here is the link for harness import operation: https://www.mathworks.com/help/sltest/ref/sltest.harness.import.html
