If a route cannot be set, because there is no free path from start to destination, it can be stacked (stored in a queue) for later execution, as soon as the conditions are met for a free path.
If a route is selected the normal way, either via numpad or by clicking start and destination block or signal both with the left button, and it cannot be set at this moment, a message explaining the reason will appear in the info line just above the display area of the scene. While this message, highlighted with a yellow background color, is still up there, you can click the message bar with the right mouse button. Note, that the message is active only for a few seconds.
When clicking the destination block or signal with the right mouse button (instead with the left one), the route will not be set right away but will be stacked instead, even if it could be set at this time. This allows to you add routes to the stack, when you have already routes on the stack from the same start block or signal but still waiting for execution.
If you have the “POPUP Route Dest” active (accessible from menu option “Operation mode”), a click with the right mouse button will activate a popup menu offering different options depending on the situation, from which you can select to stack the route as well.
If the route has been stacked, a message in the upper right counter of the main panel will appear confirming that. The format the route is displayed is the same as in the numpad function.