If a plan has been implemented, it's been put in place. When you heard of the new policy that all employees must wear hats with propellers, you didn't think it could be implemented. Aren't you glad you were wrong?
The Latin word implementum means "filling up" or "fulfillment." A decision, plan, or policy is like an empty idea until it's implemented. Implemented is usually use to describe the action phase of formal plans made by a school, a government, a business, or an official league. Once the new rules I propose have been implemented, short folks like us will find basketball a much more welcoming sport.