Minecraft wiki entry for fill, associated text: destroy - Replaces all blocks (including air) in the fill region with the specified block, dropping the existing blocks (including those that are unchanged) and block contents as entities as if they had been mined with an unenchanted diamond shovel or pickaxe. (Blocks that can be mined only with shears, such as vines, do not drop; neither do liquids.)
hollow - Replaces only the blocks on the outer edge of the fill region with the specified block. Inner blocks are changed to air, dropping their contents as entities but not themselves. If the fill region has no inner blocks (because it is smaller than three blocks in at least one dimension), acts like replace.
keep - Replaces only the air blocks in the fill region with the specified block.
outline - Replaces only the blocks on the outer edge of the fill region with the specified block. Inner blocks are not affected. If the fill region has no inner blocks (because it is smaller than three blocks in at least one dimension), acts like replace.
replace - Replaces all blocks (including air) in the fill region with the specified block, without dropping blocks or block contents as entities. Optionally, instead of specifying a data tag for the replacing block, block ID and data values may be specified to limit which blocks are replaced.
If not specified, defaults to replace.