для начала стоит прочитать про этот буфер в мануале.
The Special Buffers function works in conjunction with certain image
or pixel filters and lets you make surface-by-surface adjustments.
Essentially, the settings are arbitrary values associated with a surface
and can be used any way the filter wants. For example, the Corona image
filter can use special buffers for its Threshold Mask setting.
When you click the Special Buffers button, a dialog will appear with
four input fields. The values you enter depend on what the filter seeks.
NOTE
Use values from 0 to 1 for older filters requiring 0 to 255 integer
values.
The Special Buffer 1 field relates to the first filter in the image or
pixel filter list on the Processing tab of the Effects panel. Special Buffer
2 relates to the second image filter, and so on. The Special Buffer 4 field
works on the fourth filter and all that follow it.