SC70 footprint layout
BSC (Basic Spacing between Centers) is a term that appears on IC package drawings in reference to dimensions between pins. "Basic" spacing is nominal and can change with conditions.

REF A reference dimension is a dimension on an engineering drawing provided for information only. Reference dimensions are provided for a variety of reasons and are often an accumulation of other dimensions that are defined elsewhere (e.g. on the drawing or other related documentation).

My recommendation is to start with making the CAD footprint using the dimensions on that drawing and see how it works in the assembly soldering process. The pad sizes sometimes need adjustment or the solder mask openings need adjustment as the overall artwork is optimized for re-flow soldering.