"Every Mission requires objectives to be accomplished in order to succed; usually such objectives are divided into two categories: this is the first one. Primary objectives are the goals that must be considered more important and first in the priority line; the mission won't be considered successful until all Primary Objectives aren't fullfilled, likewise, if you fail such objectives (i.e.: the objective states "Avoid Melissa's death" and Melissa dies), your mission will be automatically considered concluded and failed. Once these goals are completed, you can return to base and thereby concluding the mission, or you can stay on the field and try fullfilling the Secondary Objectives too."