Aimed effects are intended to result from an implemented policy. Side effects are those that also occur due to implementation, but were not aimed at. These effects might be both positive or negative For example, if a beach nourishment is one of the alternatives for combating chronic erosion (= aimed effect), a wider beach will be the result (= side effect). This is a positive development for the beach recreation (a positive side-effect). However, also more sand will be blown inland in the agricultural area behind the coast (a negative side-effect).
Very often, side effects are in areas of interest which are not in the direct field of the agency or ministry responsible for the project. But mostly civil servants of this ministry determine whether these side-effects are to be included in the study or not. It is clear that this can not be done without consultation of the parties involved (like farmers, fishermen and nature conservancy agencies).