What's the actual difference between a cluster and a region as global placement grouping mechanisms?

Short Answer

A cluster is a group of cells placed near each other, with location undefined until placement completes -- rarely used now that interconnect-driven placement matured. A region is similar but its location is defined BEFORE placement -- a soft region is a physical constraint with a boundary that may still change during placement; a hard region has boundaries cells cannot cross. Regions are further exclusive (only assigned cells allowed) or non-exclusive.

Technical Explanation

• A cluster is a group of standard cells placed near each other, with location left undefined until placement completes -- it resembles a module definition in the netlist.
• Clusters are rarely used today since interconnect-driven placement algorithms matured and largely superseded the need for them.
• A region is similar in spirit but its location IS defined before placement starts -- a real, positioned constraint, not just a grouping hint.
• A soft region is a physical constraint assigning a logical module to a location, with a boundary that MAY change during placement.
• A hard region has hard boundaries standard cells cannot cross -- used mainly for timing issues, grouping clock/voltage/threshold-voltage domains.
• Regions are further exclusive (only the assigned cells may be placed there) or non-exclusive -- a hard exclusive region with a predefined boundary can effectively enforce separate floorplan blocks.

Follow-up Question & Model Response

"Why would a hard exclusive region specifically be useful for separating different voltage domains within one core?"

Candidate Model Response: Because a hard exclusive region guarantees both that the assigned cells stay within a defined physical boundary AND that no other cells intrude into it -- exactly the two guarantees a voltage domain's physical isolation needs, which a soft or non-exclusive region wouldn't provide.