The map layer Fill Rule defines the filling of the map shapes according to their Value (remember that in the Layer - Map properties you can specify a column from the joint table, from which the map to take a value for each map shape). This fill rule has the following properties:

Mode - defines the mode in which the shapes are filled. Possible values are:

None - the map shapes are not filled with a fill rule

Two Color Gradient - the map shapes are filled with interpolated colors  that are obtained from a gradient defined by two colors.

Three Color Gradient - the map shapes are filled with interpolated colors that are obtained from a gradient defined by three colors.

Distinct Values - the map shapes are filled with distinct fillings that are obtained by a palette.

Range Count - visible only when Mode is set to Two Color Gradient or Three Color Gradient. Defines the number of ranges in which to split the values set. The size of each range depends on the values set and the Distribution property. These ranges can be shown in the map layer legend - see Layer - Legend) for more information.

Distribution - visible only when Mode is set to Two Color Gradient or Three Color Gradient. Defines the way in which the values in the values set are distributed in the ranges. Possible values are:

Equal - also known as quantiles, this method allows for unequally sized data intervals and involves adjustment of the interval limits until an equal number of data points can be slotted into each interval.

Equal Interval - also known as equal ranges (or steps), this method involves division of the entire data range into equally sized intervals.

Optimal - adjusts the size of data intervals in order to minimize the classification error and thus the map looks more balanced and the results seem more correct, with just a few shapes in the highest class as one would expect.

Colors - visible only when Mode is set to Two Color Gradient or Three Color Gradient. Defines the From Color and To Color of the color gradient. In case Mode is set to Three Color Gradient also defines the middle color of the gradient.

Palette - lets you choose the palette from which to take filling entries in case the Mode is set to Distinct Values. Each distinct value in the values set is displayed with an entry from this palette.