Hi
Sequence means that there is a definite pattern to a child developing e.g. a toddler being able to walk before they can run.
Rate means the speed in which a child develops.
When a child develops if they achieve this by sequencing it enables you to plan effectively and at the right time.
When recording the rate of development it helps you to identify any concerns that you may have within the development area, this enables you to further investigate why this is happening.