The drain state allows rows in the queue tables to be drained. When you stop the Change Selector or it is in a drain state, data is still being captured in the logs, but is not being read. The tables are not capture enabled and the Change Selector is waiting for something to do.
When you start the Change Selector again with a warm start, it resumes at the point in the logs where it left off. Cold start is not recommended.
Note: Do not use or change source tables while the Change Selector is drained or stopped.