"Cutoff" values in iterative deepening search

Question

I am trying to finish my AI assignment with iterative deepening search. But when I expand the code, I can't understand what value "cutoff" variable has so I can't continue my homework. My teacher refers and teaches us the algorithm from "Russell 2016 Artificial intelligence a modern approach". Please help me. Thank everyone.

`def depth_limited_search(problem, limit=50):
    """See [Figure 3.17] for the algorithm"""
    return recursive_dls(Node(problem.initial), problem, limit)

def recursive_dls(node, problem, limit) :
    cutoff = ??? ; 
    if problem.goal_test(node.state) :
        return node
    elif limit == 0 :
        return cutoff
    else :
        cutoff_occurred = False
        for child in node.expand(problem) :
            result = recursive_dls(child, problem, limit - 1)
            if result == cutoff :
                cutoff_occurred = True
            elif result != None :
                return result
        if cutoff_occurred :
            return cutoff
        else :
            return None   

''' IMPLEMENT THE FOLLOWING FUNCTION '''
def iterative_deepening_search(problem):
    """See [Figure 3.18] for the algorithm"""
    depth = 0
    while depth <= float('inf') :
        result = depth_limited_search(problem, depth)
        if result != cutoff :
            return result`