Question:

What does the Default Height define for a Subsurface Utility Node Feature Definition?

Answer:

This defines the default height when placing a node from the top of the inlet to the invert elevation. If the default height is less than the height defined by the cell, it will place the invert as the height of the cell.

Example:

Here are the Top and Bottom cells that define the Inlet Flush Grate. Boxed in red is the elevation of the top of the inlet as 1.64 feet:

Boxed in red below is the invert elevation as 2 feet:

If you add these together it is 3.64 feet. The Feature Definition states that the Default Height is 1. Since this is less than 3.64 feet, the node's invert elevation will be placed 3.64 feet below the defined inlet elevation. This node was placed with a 0 inlet elevation. When you open the Utility Properties, you can verify that the invert elevation is set to -3.64 feet: