Plan Your Timetable is a planning tool that allows you to experiment with different combinations of classes (e.g. lectures/tutorials) to come up with a timetable that suits your availability. You cannot do this in eStudent, as it only allows you to see the classes for units that you are already enrolled into.
Planning a timetable in Plan Your Timetable does not affect your enrolment or class registration. You will still need to go into eStudent to complete these steps.