This is the depth of only one layer. All layers - are located in order from the surface, on least their location in .xml.
<layer depth="2" blockname="terrDirt" />
<layer depth="4" blockname="terrClay" /> <resource blockname="terrGravel" cluster="0" prob="0.05" rwgGenerationType="all" /> </layer>
<layer depth="3" blockname="terrSand" />
<layer depth="*" blockname="terrStone" />
<layer depth="3" blockname="terrBedrock" />
this means (from the beginning of the surface):
first is a 2 blocks of "dirt", after - 4 "clay" mixed with "gravel" blocks, 3 "sand", all the other(*) blocks before bedrock - "rock", and at the bottom is 3 blocks - "bedrock".
After the "bedrock" - blocks can not be, it is always located at a world depth = 0 (more precisely 0-2, if a depth= 3).
The world limit is 256 blocks.