Metasploit-EXITFUNC

This EXITFUNC option effectively sets a function hash in the payload that specifies a 
DLL and function to call when the payload is complete.

There are 4 different values for EXITFUNC : 
    + none
    + seh
    + thread 
    + process. 
 
 
Usually it is set to thread or process, which corresponds to the ExitThread or ExitProcess calls. 
"none" technique will calls GetLastError, effectively a no-op. 
The thread will then continue executing, allowing you to simply cat multiple payloads together to be run in serial.


EXITFUNC will be useful in some cases where after you exploited a box, you need a clean exit, 
even unfortunately the biggest problem is that many payloads don’t have a clean execution path after the exitfunc.


- SEH:
     This method should be used when there is a structured exception handler (SEH) that will restart the 
     thread or process automatically when an error occurs.

- THREAD:
      This method is used in most exploitation scenarios where the exploited process (e.g. IE) runs the shellcode 
      in a sub-thread and exiting this thread results in a working application/system (clean exit)
      
- PROCESS:
      This method should be used with multi/handler.
      This method should also be used with any exploit where a master process restarts it on exit.
     
       

Source: https://www.hacking-tutorial.com/tips-and-trick/what-is-metasploit-exitfunc/