That’s how they are called in the GIMP. Other image editing applications have similar features by other names, but they work very similarly.
This are features Synfig needs. In order to do the same on Synfig one needs to use the Canvas properties window unintuitive UI. While it is possible to do the same it’s far more complicated.
So, the idea is to have a Canvas Expand and Canvas Resize options which do same operations I explained on linked post, but without having to mess around with the “Locks and Links” checkboxes (which nobody seem to understand ). Instead it works just like it works on image editing software.