Comment by sillywalk

Not sure how much this has changed since 2011..

From one of John Siracusa Mac OS X Reviews @ Ars Technica[0]:

"Sudden Termination, a feature that was introduced in Snow Leopard, allows applications to indicate to the system that it's safe to kill them "impolitely" (i.e., by sending them SIGKILL, causing them to terminate immediately, with no chance for potentially time-consuming clean-up operations to execute). Applications are expected to set this bit when they're sure they're not in the middle of doing something, have no open files, no unflushed buffers, and so on.

This feature enables Snow Leopard to log out, shut down, and restart more quickly than earlier versions of Mac OS X. When it can, the OS simply kills processes instead of politely asking them to exit. (When Snow Leopard was released, Apple made sure its own applications and daemon processes supported Sudden Termination, even if third-party applications didn't.)

Lion includes a new feature called Automatic Termination. Whereas Sudden Termination lets an application tell the system when it's okay to terminate it with extreme prejudice, Automatic Termination lets an application tell the system that it's okay to politely ask the program to exit.

But wait, isn't it always okay for the OS to politely ask an application to exit? Isn't that what's always happened in Mac OS X on logout, shutdown, or restart? Yes, but what makes Automatic Termination different is when and why this might happen. In Lion, the OS may terminate applications that are not in use in order to reclaim resources—primarily memory, but also things like file descriptors, CPU cycles, and processes. Advertisement

You read that right. Lion will quit your running applications behind your back if it decides it needs the resources, and if you don't appear to be using them. The heuristic for determining whether an application is "in use" is very conservative: it must not be the active application, it must have no visible, non-minimized windows—and, of course, it must explicitly support Automatic Termination.

Automatic Termination works hand-in-hand with autosave. Any application that supports Automatic Termination should also support autosave and document restore. Since only applications with no visible windows are eligible for Automatic Termination, and since by default the Dock does not indicate whether or not an application is running, the user might not even notice when an application is automatically terminated by the system. No dialog boxes will ask about unsaved changes, and when the user clicks on the application in the Dock to reactivate it, it should relaunch and appear exactly as it did before it was terminated."

[0] https://arstechnica.com/gadgets/2011/07/mac-os-x-10-7/8/