Often, the significance will also relate to impacts on aboriginal communities, and that is determined through discussions with the aboriginal communities and the proponent in terms of what the expected effect will be.
I'm not an EA science practitioner, so I won't get myself into scientific details. I could ask for a follow-up, if you like. In terms of significance, it's normally about the scope and scale. For example, a mine can remove the top layer of earth of hundreds of square kilometres or square miles, compared to in situ, where the impacted area is relatively smaller.