What is the meaning of the parameter "chain" in utils.facet module?

I have found the parameter "chain" from the yade.utils module as follows;

https://yade-dem.org/doc/yade.utils.html?highlight=utils%20facet#yade.utils.facet

However, I cannot find the meaning of "chain" from the utils. sphere's document
Please tell me the meaning of the parameter "chain"

Regards,

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yade.utils.facet(vertices, dynamic=None, fixed=True, wire=True, color=None, highlight=False, noBound=False, material=-1, mask=1, chain=-1)
Create a Facet-shaped body with given parameters. Body center is chosen as the center of the inscribed circle of the vertices triangle

Parameters:
vertices ([Vector3,Vector3,Vector3]) – coordinates of vertices in the global coordinate system.
wire (bool) – if True, facets are shown as skeleton; otherwise facets are filled
noBound (bool) – set Body.bounded
color (Vector3-or-None) – color of the facet; random color will be assigned if None.
See utils.sphere’s documentation for meaning of other parameters.

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