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The reason you never heard about it, is not a term in PCB design.
Unfortunately the poster has not indicated the question is related to a specific tool in this case Allegro. There must be other tools that use similar or the same terminology.
After version 14.2XX Cadence introduced Dynamic shapes in Version 15.0X and up. Dynamic shapes allows users to edit, move elements like traces, vias and component pads on different net within DS and shape automatically creates voids or clearance as oppose to static shape which requires user to refresh or manually creates clearances.
I use mostly DS.It is much easier to work with. And use Static Shapes in areas that do not need much editing.