A child will usually progress from one milestone to the next, normally one accomplishment has a bearing on the following and if a child has difficulty meeting a milestone it can mean delay in other areas also.
The sequence of development is not to be confused with the rate of development as every child will progress and grow at different speeds. A child could accomplish milestones quicker in one area say language such as speaking than another child but may take more time to accomplish a physical milestone such as walking.
The rate of development describes how quick a child takes to accomplish these milestones as the speed can vary from child to child, for example one child could learn to walk at 11 months whereas another could achieve walking at 14 months.
It is importance to identify the difference and how each one plays a role in identifying the accomplishments of a child, what milestones have been reached and when. They enable charting of a child or young person's development to happen and provide a structure or picture that can measure where a child might be in need of support or extension.