I've been doing some review of particle physics in Griffiths and I came across this statement that really didn't stand out to me before. "the idea is to draw all the diagrams contributing to the process in question (up to a desired order), calculate the amplitude (M) for each one, and add them up to get the total amplitude" I would like to know what is the significance of the "up to a desired order" statement? I've read in books about quantum field theory that the idea is we are actually using a perturbation procedure in the Feynman diagram formalism. Is there any evidence to that claim or am I confusing it with something else? Or instead is the order of a process just have to do with the number of external/internal lines in a diagram?