(a) This portion of the rule shall be applicable to acute care hospitals, as defined in paragraph (f) of this section: Except in extraordinary circumstances and in circumstances in which there are existing non-conforming units, the following shall be appropriate units, and the only appropriate units, for petitions filed pursuant to section 9(c)(1)(A)(i) or 9(c)(1)(B) of the National Labor Relations Act, as amended, except that, if sought by labor organizations, various combinations of units may also