Polychords are when multiple chords (particularly triads) are played at the same time. They are used to expend the harmony and offer an interesting blend of acoustic colors.
While there is no limit to the combined chord qualities of the number of chords used to build a polychord, typically people make polychords just out of two triads.
Below is a representation of a poly chord with the proper notation (similar to a fraction sign):
Notice how this polychord above can also be written as a Bb13 chord with no 7th.
That being said, some polychords can ultimately be jazz chords (and this term is used lightly, as no chord belongs exclusively to a genre of music) with upper extensions. So, that is up to you, the composer, to play around with triadic combinations.
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