The current thread is defined as the flow of execution invoked by the most recent hotkey, timed subroutine, custom menu item, or GUI event. That thread can be executing commands within its own subroutine or within other subroutines called by that subroutine.
Although AutoHotkey doesn't actually use multiple threads, it simulates that behavior: If a second thread is started -- such as by pressing another hotkey while the previous is still running -- the current thread will be interrupted (temporarily halted) to allow the new thread to become current. If third thread is started while the second is still running, both the second and first will be in a dormant state, and so on.
When the current thread finishes, the one most recently interrupted will be resumed, and so on, until all the threads finally finish. When resumed, a thread's settings for things such as ErrorLevel and SetKeyDelay are automatically restored to what they were just prior to its interruption; in other words, a thread will experience no side-effects from having been interrupted (except for a possible change in the active window).
Note: The KeyHistory command/menu-item shows how many threads are in an interrupted state and the ListHotkeys command/menu-item shows which hotkeys have threads.
A single script can have multiple simultaneous MsgBox, InputBox, FileSelectFile, and FileSelectFolder dialogs. This is achieved by launching a new thread (via hotkey, timed subroutine, custom menu item, etc.) while a prior thread already has a dialog displayed.
By default, a given hotkey or hotstring subroutine cannot be run a second time if it is already running. Use #MaxThreadsPerHotkey to change this behavior.
Any thread (hotkey, timed subroutine, custom menu item, etc.) with a priority lower than that of the current thread cannot interrupt it. During that time, such timers will not run, and any attempt by the user to create a thread (such as by pressing a hotkey or GUI button) will have no effect, nor will it be buffered. Because of this, it is usually best to design high priority threads to finish quickly, or use Critical instead of making them high priority.
The default priority is 0. All threads use the default priority unless changed by one of the following methods:
1) A timed subroutine is given a specific priority via SetTimer.
2) A hotkey is given a specific priority via the Hotkey command.
3) A hotstring is given a specific priority when it is defined, or via the #Hotstring directive.
4) A custom menu item is given a specific priority via the Menu command.
5) The current thread sets its own priority via the Thread command.
The OnExit subroutine (if any) will always run when called for, regardless of the current thread's priority.