my understanding is that it is when the plant alternates the growth side.

at first the plant produces two fan leafs per node, with each node at 90 degrees to the one b4. (not alternating)

once the plant is mature, the nodes alternate. So one fan leaf will be produce on one node. The next node the fan leaf will be produced on the other side of the plant. so the fan leafs alternate sides.

not 100% sure.

google will probaly produce the correct answer.