From Organic Design wiki

In the nodal model, a loop is a circularly linked list formed from nodal associations. Since none of the nodes in a loop is a starting or finishing point, a loop can be referenced by any of its node-items. This means that a reference to a loop can also encapsulate the concept of an item of current focus, a concept used heavily by nodal reduction.

If a loop is not circularly-linked, then the last item must link to root and it is called a thread instead. Threads only rotate once and are then automatically unhooked because every quanta sent to root is a new quanta.

In nodal reduction, a node's value (obtained by calling nodeGet/SetValue with the key parameter set to zero) is called the current focus and it is a node in the loop. This node of current-focus is the one which will receive the next quantum of execution from its parent node, and is part of a loop of nodes which all receive quanta as the loop is rotated by the nodal reduction process.

If a loop has static structure, then it can appear in many contexts simultaneously, it's parent association will be dynamically maintained by the nodal reduction algorithm.