Some exercises may either be time, reps, distance, etc depending on how you're training them. E.g. you may do squats for time in a circuit or reps for strength.

Exercises shouldn't be categorised by reps, time etc from the start necessarily but changeable depending on the nature of the training. This allows better tracking also, since you can see if a client did 30s of squats or 10 reps (I have clients who often get confused with tracking time and end up putting down 45 push ups instead of the reps in 45s when 45s is programmed in). TrainHeroic, for example, allows this.