Creating half normal probability distribution I have come across a problem where a half normal distribution is based on a single number, namely the sum of all costs. The exact definition of the number is not important. The important think is that it is a single number. I also know how many individual costs have been used to get to the total. Ex. there were 200 observations and the grand total is 25000.
This problem is in Bayesian context. A half normal distribution is used as weakly informative prior and the only input for the half normal distribution is the grand total. I cannot understand how this half-normal distribution is created solely with the grand total and the number of observations? Am I missing something?
Here the distribution is called. And here is explained that the grand total is to be used as input for the half normal distribution. I cannot recall the exact location, but somewhere in the docu I have read that the scale parameter is 2.
I feel like I am missing something with respect to the creation of the weakly informative prior. Is some sampling used or something? Here is the source code that is used for the half normal distribution.

Normal distribution is parametrized by two parameters - location $\mu$ and scale $\sigma$. Half-normal is parametrized by only one parameter - its scale $\sigma$, since the location is fixed to $\mu=0$ by the very definition of this distribution.
