[IBIS] difference between [Pullup Reference] and [Voltage Range]

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Hello Friends,

 As I read in IBIS standard

Voltage Range----Defines the power supply voltage tolerance over which 
the model is intended to operate.
Pullup Reference--Defines a voltage rail other than that defined by the 
[Voltage  Range] keyword as the reference voltage for the [Pullup] I-V  
data.

  My buffer have dual supply VDD and VDDE2V5. VDD is the core side 
supply and VDDE2V5 voltage at the pad as well as voltage at the pullup 
and power clam diode.

 I wnat to know how this information is use in the simulation.
 When pullup information is present then it's overwrite voltag information?

I have given
[Voltage Range]  1.2 1.1 1.3
[Pullup Reference] 2.5 2.2 2.7

   Which information is used in the simulation.

Regards
Akhilesh

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