"window utilization"
Assume you put N turns of copper wire through a 'window' in the iron core. The length of that winding will be more or less independent on the area of the wire used. This means that the resistance of the winding (a bad thing) will be inversely proportional to the copper area.

A good transformer has as low a resistance as possible, so will have the largest area of copper possible. The 'windows' through the iron core obviously put an upper limit to the area of copper. The utilisation factor is just the ratio of actual copper you can get through the window versus the area of the window.

With a single turn of copper bar, you could approach 100%, but that's a rather unusual transformer winding.

With conventional round wire, with an insulation covering (doesn't count as copper), wound in single layers, with inter-layer tape, with an integral number of turns not exactly filling the width, or layers filling the height, on an insulation bobbin, getting 50% utilisation would be pretty good going, this is the figure to aim for. 40% may be more typical.

Some transformers are wound with lower utilisation, where for example the manufacturer has had to use a standard core the 'next size up', and then finds he can meet the customer's specification without filling the window, so saving some cost of the extra wire.

A high voltage transformer, that needs thick insulation around the windings, between the windings and perhaps between the layers will also have a utilisation well below 50%.

You may do a little better than 50% if you can get rid of the bobbin and use self-supporting windings (like most microwave oven transformers), or use flat tape or square wire to wind with, which reduces the wasted corners between round wires, or choose the wire size carefully to fit an integer number of turns and layers in the window.

With a toroidal transformer, the winding machine needs a hole left in the middle to work through, which further reduces the available window, and 40% is good going.

For two transformers wound with the same number of turns on the same cores, but with different utilisation factors, the one with the higher factor will be ...
more efficient
higher current capability
lower losses
better regulation
heavier
more expensive