Remember "interface" and "interior" are different. For the former, it's an external boundary so only c0 exists, for an interior c0 and c1 are available.
Example 3, https://ansyshelp.ansys.com/account/Secured?returnurl=/Views/Secured/corp/v221/en/flu_udf/flu_udf_sec_define_dpm_bc.html shows it in use.
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