Any window that is not tiled is assigned to the untiled subdesktop. These windows are completely separate from the tiled subdesktop. The untiled subdesktop behaves more or less like any traditional window manager, but adds a few keyboard shortcuts for moving windows to different parts of the screen or zooming them in both dimensions.
larswm will not tile a window unless its class/instance can be found in the larswm.dotileclass list.
The way to move between your tiled and untiled desktop is through a keyboard shortcut, which by default is Shift-Control-BackSpace
When a window mapped on the untiled subdesktop for the first time, it is centered on the screen, unless it has had its position set in the hints, in which case that position is used instead of centering.