can anyone tell me what really difference between boundary surface and fill surface?

This is taken from the SW 2014 Surface Modeling training book.

"The boundary surface is limited to four-sided patches. It uses pairs of parallel edges in two directions. Most importantly, the boundary surface feature lets you create surfaces that can be tangent or curvature continuous in both directions (all sides of the surface)."

"The Filled Surface feature constructs a surface patch with any number of sides, within a boundary defined by existing model edges. sketches, or curves. Filled Surface requires a boundary of surface edges or sketch entities. It may even work without a closed loop. If surface edges are selected, boundary conditions such as Contact, Tangency, or Curvature may be selected. The Filled Surface can knit itself into the surrounding surface bodies, knit an enclosed volume into a solid or integrate itself directly into a solid body."

Hope this helps.

Russell