Unless you are using the output of F_C0 to obtain the cell centroid with C_CENTROID(xc, c, t), F_C0/F_C1 return cell_t for cell c, on their respective side of the face. F_C0/F_C1 does not return coordinates. cell_t is an integer data type that identifies a particular cell within a cell thread.
Similarily for THREAD_T0, which returns a cell thread pointer for the cell adjacent to the cell. If the cell on the other side of that face does not belong to the same cell thread, THREAD_T1 will return the corresponding cell thread pointer for that cell.
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