Adobe ACROBAT 3D Manual page 610

Setting trap widths
Differences in paper characteristics, screen rulings, and printing press conditions require
different amounts of trap. Each Trap Width control allows a maximum value of 8 points.
To determine the appropriate trap widths for each job, consult with your prepress service
provider. Trap presets provide two different settings for trap width (the amount of overlap
for each trap):
Default specifies the trap width in points for trapping all colors except those involving
solid black. Enter values from 0p0 to 0p8. The default value is 0p0.25.
Black indicates the distance that inks spread into solid black, or the holdback amount--the
distance between black edges and underlying inks for trapping rich blacks. The default
value is 0p0.5. This value is often set to be 1.5 to 2 times the value of the default trap
width. The value you set for Black Color (in the Trap Thresholds section) determines what
Acrobat will consider to be a solid black or a rich black (a process color consisting of
solid black with one or more components of C, M, or Y inks). For details, see Trapping
with black and rich black.
Image specifies the trap width in points for trapping all images. Enter values from 0p0 to
0p8. The default value is 0p0.25.