Can you tell me what does the option "zero" in genpoisson{VGAM} mean?

 zero An integer vector, containing the value 1 or 2. If so, *ë* or
*theta*respectively are modelled as an intercept only. If set to
NULL then both linear/additive predictors are modelled as functions of the
explanatory variables.

To fit a simple model like log(theta/(1-phi))=bo+b1x, which value of zero
should i use 1 or 2 or NULL? Please help me its urgent!

Thanks!

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