The active structure is described as undirected because the links in its structure can propagate information in either direction. This allows the same node to be either an input or output. It also allows information to arrive at an operator along a link, and the operator immediately return new (consistent) information along the same link.
Some connections are directed - the Start pin on a FOR operator, and some operators constrain direction - the ':=' or assign operator, for example.
It is possible to set a link to behave uni-directionally, and even to "fix" a link, so information can travel in neither direction.
These are examples where local modelling overrides the basic system behaviour, but undirectedness is necessary to model behaviour in general, and is essential for Constraint Reasoning.
Undirectedness allows a system to break its apparent hardware limitations. As a crude example, consider you have a model with one thousand statements. Each statement is directed to a purpose and has one use, giving us 1000 * 1E1000, or 1000 uses. If each statement can have, on average, three uses and those three uses occur within one hundred statements, we have 1000*3E100, a very large number. If a system can achieve it, undirectedness offers a huge increase in capability. It also allows the creation of free cognitive structure.
See Free Structure