There are two states of the signal polygons: Unfilled and uncomputed or filling computed. The first state is a bit imprecise but flexible, the second state is exact but cemented.

In the state "unfilled" or "uncomputed" the ground polygons are represented as a kind of sandwich with an optical trick: The area is drawn first, above it the auras (=spacing) of the traces in background color and above it finally the traces. The sandwich looks like this:

(Image 1: Example signal polygon for "copper bottom")

If an "rival" signal track is changed now, its aura moves directly along with it. So the uncomputed state is very fast and flexible. The ground plane is not disturbing during the design flow. Furthermore you can easily hide the corresponding layers (here 0 + 1) and the polygon is not visible at all during layouting. You can easily reach this state typing the [<] key: The area becomes "uncomputed" and invisible because the lower layers are hidden.

The disadvantage of this state is that the areas are not exact. Corners are really sharp - but during production a radius will result here. Very thin lines will be displayed, although they could not be produced or only incorrectly. Islands that are not connected remain and would create unwanted capacities on the PCB.

The state "filled" or "computed" on the other hand is very exact and shows the ground plane as it will be produced finally. Also the ligaments for thermal pads are now computed exactly and - if desired - the unconnected islands are removed. Unfortunately, the calculation of complex areas takes some time and the surface is now cast in concrete. Changing "rival" tracks now apparently leads to distance violations or short circuits. Typing the key [>] you can recalculate the polygon at any time. Nevertheless the state "filled" during the routing can be very hindering.

Conclusion: "Uncalculated" is fast and flexible. The [<] key hides the areas briefly. "Filled" is exact. The [>] key recalculates and fills all visible signal polygons.

(Image 2: Unfilld and uncomputed: Unproducible thin lines and sharp corners)

(Image 3: Filling calculated: Rounded corners, unconnected islands are removed)

All signal polygons are recalculated before each output for production and before calculating the airwires and before the DRC (design rule check).