concept of tangent and residual in AceFem and automation

here trying to solve the pde: ut + ux = 0 in acefem. I have two questions regarding tangent and residual.

1. Tangent shows the s$$ and residual shows p$$ in the acefem code.
   There are two parts comes under p$$ and s$$ which are connected through p$$, "AddIn" -> True and s$$, "AddIn" -> True respectively.
   Here I could not able to understand that what does this " "AddIn" -> True " does to the p$$ and s$$? Does it shows the direct addition of first part and second part of p$$?

   2. How exactly SMSStandardModule["Tangent and residual"] works in acefem. I have already checked with acefem help in mathematica. It seems to me that it operates upon default standard user subroutine for tangent and residual. If yes then I would like to see that how does the subroutine looks and works.
      I also checked the default acefem codes for solving the pde with finite element method.
      There, by directly using SMSStandardModule["Tangent and residual"] will gives the solution. But in the case of above mentioned pde, I need to provide each entry of p$$ and s$$ manually. So is there any way to create p$$ and s$$ automatically?

   Your suggestion’s to make it automaticlly would be helpful to understand the concept.
   Thank you.