How to understand pub variables

[Xiang Liu]

Dear all,

My name is Xiang and I am a beginner of learning Tamarin. I am a little confused about how to understand the meaning of a pub variable, e.g. $A. For example, in the tutorial, the rule to associate A and its priv-pub keys is given as follows:

Rule Register_pk:

          [Fr(~ltk)]-->[!Ltk($A,~ltk), !Pk($A, pk(~ltk))]

What's the difference between the above rule and 

Rule Register_pk:

          [Fr(~ltk)]-->[!Ltk(A, ~ltk), !Pk(A, pk(~ltk))]

Thanks.