When Importing a STEP file the warning message "Assemblies are currently flattened; structure may change in the future" displays in the Property tab.
This warning most commonly appears if the original CAD file was an assembly. nTop doesn't support assembly files, but we do support "multiple-body" parts. The geometry is unchanged but the process modifies the file structure.
This message lets you know something changed, but it shouldn't cause any issues. If you would like to avoid this warning there may be an option to change export behavior in the CAD tool. Alternatively, try importing a Parasolid (.x_t).