I hope I can answer this as coherently as possible. According to the PMBOK (5th Ed), Leads and lags are tools and techniques for most of the Schedule Management Plan, but they are always applied during network analysis. During Sequence Activities, a Lead/Lag is applied based on relationship and dependencies of two activities. During Develop Schedule, these leads/lags contribute to the overall viability and optimization of the schedule. During Control Schedule, leads/lags become tools to bring activities that are behind schedule back into alignment with the schedule plan.
Since the Control Schedule process is used to "recognize deviation from the plan and take corrective and preventative actions," lead and lags are great tools to monitor and control ACTIVITY timelines. However, it is critical (especially for the PMP Exam) that you remember that "change to the schedule baseline can only be approved through the Perform Integrated Change Control process," which is initiated as a CHANGE REQUEST; an OUTPUT of the Control Schedule Process.
So, in answer to your question, leads and lags are used as tools in the Control Schedule process, and may result in a change request. This change request becomes an input into the ICC process and may result in an adjustment to the schedule.
Really hope that helps clear it up!
