If we invert an Octave dyad—raise or lower either of its notes by an octave—we end up with a Unison: two instances of exactly the same pitch. Although included with the Harmonies, the dyad of the Unison is technically not a harmony at all: there is no open space within it, no difference in frequency.
In a casting the Unison indicates unity. Its extreme consonance points toward ease between that which is unified. If the Unison is playing out unconsciously, however, it can indicate codependency or enabling.
In the active suit of Brass, the hermeneutic for the Unison is collaboration; in the emotional suit of Strings, empathy; in the intellectual suit of Woodwinds, agreement; in the physical suit of Percussion, partnership; and in the spiritual, holistic suit of Voices, fellowship.