There's no stable unit for the resolution.
As far as I understood (maybe this has to be corrected):
The resolution value means a 3D grid resolution.
DAVID constructs a "bounding cube" around your object. Its side length is 1.2 times the biggest dimension (either in X, Y or Z)
Means if there is a object of 100x50x50 mm the bounding cube is 120x120x120 (1.2 x 100).
This bounding cube is devided by the resolution value.
If you set Resolution to 250 there are 250x250x250 cells. Each cell cube is 0.48x0.48x0.48 ( 120/250 - its side length is displayed as "VertexSpacing" right below the Res slider)
Each cell which is intersected by the surface to fuse constructs a vertex of the fusion result. The vertex isn't in the cells center but at a 3D place which represents the surface best.
Edit: Sorry, there was a mistake saying that "120/250=0.12". The red values are corrected now - Thanks, Erhardt!