Adobe After Effects CS3 - Manual (Page 277)

AFTER EFFECTS CS3
User Guide
272
Gray Level Softness Specifies how soft to make the edges of the matte. At 0%, the matte edges contain only fully
opaque and fully transparent values. At 100%, the matte edges have a full range of gray values but may appear
blurred.
3 Set stage-two controls (the next three properties) to choke the matte by the same amount as that by which you
spread it in stage one.
4 (Optional) Use the Iterations property to specify how many times After Effects repeats the spread-and-choke
sequence. You may need to try a few different settings so that the sequence is repeated as many times as necessary to
close any unwanted holes.
See also
“Keying overview and tips” on page 268
“Matte effects” on page 450