The links are the GINC Trade Dependency Graph’s: every product is joined to every product it is a direct input to or made from, the strongest links drawn first. What the graph is.
Each heading’s links are worked out from its products’: the input share from heading A to heading B is the sum of the input shares from A’s products to B’s, divided by the number of products in B, so a heading’s input shares add up to 100%, as a product’s do; the output share is worked out the same way. Two headings are as close as the larger of the two.