SysRq / Break / Pause - function and using ?

[MIRKOSOFT]

Hi!

I want to remap key to one of following;
SysRq
Break
Pause

'cause emulator which I'm using for retrocomputing have no other key free.

What is exact their function i current real Windows (newer than Vista) and what was in Windows before 95 or even in DOS?
SysRq - it's subkey of PrintScreen (we all know that PrtScr currently does screen/window capture and in early Windows and DOS versions allowed output to printer), but is SysRq abbreviation of System Request?
But - if we look at keyboard - how to press this function - Shift does nothing or its function is not more supported, no matter - what is and was its function - by name it looks like performing any request to OS, so please explain me its function in present and past and how to press this key or combination to send this command.
Break - it looks like break system operation by name - if it was so, it is past - so what was/is its function? Even I saw in emulators also Keyboard input > Send Ctrl+Break - here I don't know what it means completely and how to activate this function or key...
Last key - Pause - it's function again looks like OS pause by name and also looks like in old OSes, but how to activate it? Even it looks like is hardwired on keyboard with NumLock - is it truth or not? Remapping software always at change of this key affects also NumLock function...

Can anybody explain me these mysterious keys and their using, and also using in AHK?
Thank you for each reply.
Miro