Wonder why booster shots leave a scar & r usually placed on the upper arm?

Placement: Shots are given in the upper arm because it is a convenient place where there is a big muscle in which to inject. Shots do not scar, but a vaccination can. The reason that the upper arm was chosen was not only because of the placement of the muscle but that it is a convenient site and could be a standard site when searching to see if a person has been vaccinated.