Halo (keep-out margin)
Beginner
A clear border kept around a big block so small parts don’t crowd right up against it.
Novice
A border around a large block (a macro, such as a memory) where no small cells may be placed. It is attached to the block and moves with it.
Expert
DEF COMPONENTS + HALO [SOFT] left bottom right top: a placement blockage around a macro’s edges. A soft halo is honored only during initial placement, so later buffering and clock tree synthesis may use it.