Region, guide and fence
Beginner
A fenced yard on the chip where one group of parts must stay together.
Novice
An area assigned to a group of cells. A guide is a preference the placer may break if wirelength or timing are better elsewhere. A fence is a strict rule: the group must stay inside, and no other cells may enter.
Expert
DEF REGIONS with TYPE GUIDE or FENCE, with cells assigned by COMPONENTS + REGION. Over-tight fences push local density up and starve the optimizer of space.