Bias
For some patterns, exposing the exact shapes you want to end up with isn't optimal. You might wish to compensate for an etch bias such as an undercut, or feature growth due to electron scattering during exposure. There's no need to go "back to the drawing board" and redraw your patterns at a smaller size. The BIAS module will change the size of your pattern very intelligently, either growing or shrinking your pattern.
Consider this pattern:
Now, if I wish the shrink this pattern, say make everything 50 nm smaller, I can use a BIAS module with parameters set such as:
and in the next figure, you see the result, shown in RED, overlaid on the original BLUE shapes. All edges are pulled in by the specified 50 nm size, but the biasing is done intelligently, so that internal shapes are not pulled apart, diagonals are preserved, etc. Specifically, the RED pattern is 50 nm smaller in all directions than the original BLUE pattern.
Of course, entering a positive value for BIAS will GROW the shapes, also with the same intelligence applied to the process.