You can set an existing referential to an object when several objects are in the same "real world". For example, a mesh created from a T1 MRI is in the same referential as the MRI.
Load subject01 white mesh: data_for_anatomist/subject01/subject01_Lwhite.mesh
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Put it in the previous window containing the fusion between the MRI of the 2 subjects. Notice that it is not displayed at the same place as the MRIs.
Apply the referential of subject01.nii to this mesh with right-click menu Referential => Load.