Easily turned on and off from the menu.
This option, when
unlocked, will open a file (from outside the editor, eg. explorer, cmd.., double LMB or RMB->Open with...) in last focused ST window. When
locked, new window will open for next* file.
*next or each: The newly opened window will not be locked. Or the option can be global, meaning every file will be opened in new windows.
Drag and drop will still open a file in the desired window.
This is usefull when some plain text files (*.txt, *.rtf, *.md,..) are registered to ST and you want to open them temporarily while developing your project, but you don't want them to switch view from current working file.