In Strater 2-4 and above, none of the patterns get exported to DXF files. The polygons will simply be filled with the foreground color.
In Strater 1, the vector and stock patterns are exported to DXF, but the bitmap and picture patterns are not because the R13 and R14 DXF formats do not support images. You can tell what type of pattern you have by selecting the fill pattern drop down menu. The first line at the top of the drop down shows you the fill pattern name (oulined in red below), and the second line shows you what type of pattern it is (outlined in blue below).
Updated: October 6, 2016