#External references and top-down design

One part's sketch can stand on another part's geometry — on its face or edge. An explicit external reference then appears between the parts.

#What it is for

That is how top-down work goes: the housing is defined first, and the bracket, the lid and the gasket take their dimensions from it. Retyping those dimensions by hand means keeping a second copy of one value and one day fixing only one of them.

#The reference is live and explicit

Projected geometry follows its source: move the source part and the outline goes with it, and so does everything built on it.

And the link is not silent: it is visible in the tree and can be broken with one command. After breaking, the geometry stays where it is but stops following — which is sometimes exactly what is needed, when a part goes into production and must no longer change after the housing.

#Careful with loops

If A takes its dimensions from B and B from A, the rebuild becomes undefined. The program does not let such links close, but the intent is better kept one-way: from the main to the subordinate.