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In other words, a forcimpacted structure is one where every headmate is affected by shaping in the same way and to the same extent.
For example, a sidesystem where every single headmate in it was caused by an abuser to split on purpose would be considered forcimpacted. So would a subsystem that has experienced programming and every single member of the subsystem is programmed.
While it is uncommon for something to impact everyone in a structure in the exact same way equally, it does happen, particularly in terms of situations where every headmate in an entire structure may have been made for the same reason or in the same way.
Furthermore, some structures in the system may be forcimpacted, but some structures may be forciplex outside of the scope of forcimpacted, or even "just" forcibased.