what does the "Close node from last tick" means?

[Wuqiqi123]

behavior3py/b3/core/behaviortree.py

Lines 118 to 131 in 48097b9

	# Close node from last tick, if needed
	last_open_nodes = blackboard.get('open_nodes', self.id)
	curr_open_nodes = tick._open_nodes
	start = 0
	for node1, node2 in itertools.izip(last_open_nodes, curr_open_nodes):
	start += 1
	if node1 != node2:
	break
	# - close nodes
	for i in xrange(len(last_open_nodes)-1, start-1, -1):
	last_open_nodes[i]._close(tick);

Hi, I read the code here but this confuse me, when a node is not at running state, will close it. So "Close node from last tick" is for what?