Scatter
The fill command fills an area with blocks, but there's no control of any randomness. The scatter command does this.
scatter <block> <percentage> <x1, y1, z1> <x2, y2, z2>
Technicalities
The scatter command generates a structure which is then loaded into the world with the integrity parameter. This poses a few issues that are not a part of the vanilla fill command.
Dynamic Sizing
As the structure is generated at compile-time, it doesn't support having a dynamic fill region size. The size must be known at compile-time, either by having all coordinates relative, or by having all coordinates static.
Block Data
It's much harder for the compiler to embed data into the structure file, as it doesn't have the same information that Minecraft has internally. We're looking to revisit this in the future, but for now, there is no support for block data.