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In solid modelling, a transmigration operation refers to the changes in a part that must be propagated to changes in other parts which have a dependence on the first. Put simply it means that if something is moved, other things will be affected and must also be adjusted to preserve other established relationships.
This expression came into existence when parametric modelers like SolidEdge gained acceptance. Parametric modelers generate the BREP from the CSG representation and its associations, usually in the form of LISP expressions.
Example
Suppose one has two blocks of widely different sizes. A Euler boolean operation is performed, subtracting the smaller block from a corner of the bigger one, and resulting in a block with a square corner removed. Now suppose that a user colors one of the edges of the block and stores that change. If the user goes back and moves the cutting block so that instead of cutting a side, the user instead cuts through the center of the edge, creating a two edges; the color change is then reapplied. Those subsequent changes are associated with both broken edges, rather than one of the cut edges.