Historic membership incudes any lead who as ever entered the program
current members, means current active members whether exhausted or not
Members with engaed status is just a list that includes their status
Engaged - MQL - Engaged at the MQL stage
Exhausted - they have no more content to consume in their current stream
Engaged may or may not be at MQL depending on your Status setup and flows.
Historical - Current = those in other statuses
Current=in a Stream as Member and likely receiving emails.