The training principle known as Specificity is a simple concept. For any given goal, you need to be specific in your training i.e. to be a good runner you must run! If your goal is a triathlon you would obviously incorporate all three disciplines into your training schedule.
The Specificity principle can be applied to many aspects of your training. For instance, your individual fitness level. You should only train to your own capability. 'Overloading' too often or too much can result in injury or over-training syndrome.
Personal training sessions are often designed with specificity in mind. Coming to a Personal Trainer with the goal of Triathlon in your sights means sessions will focus on strengthening the specific muscle groups that are needed for that particular sport and attention paid to cardiovascular stamina and endurance.
Although specificity is important, it is also necessary in every schedule to include exercises of a general nature. These exercises may not relate too closely to the movement of any athletic event but they do give a balanced development and provide a strong base upon which highly specific exercise can be built.