Maperitive renders symbols in a pre-defined order (called the painting cycle). For example, polygons (fill symbols) are always rendered before any line symbols, so the lines are never overlapped by polygons. This is usually the proper way to render a map, but some special situations could require symbols to be rendered in a different order.
The painting-cycle property allows you to redefine the painting cycle for a particular symbol in a rule.
target : river define line-color : #B5D0D0 line-width : 7:1;12:2;14:5 painting-cycle : 399 draw : line
specifies that the river (as a linear element) should be rendered before (below) any polygons (fill symbols are rendered in the painting cycle 400).
Here is the list of the default painting cycles used by Maperitive: