When Maya draws or renders the scene, it looks at each node in the scene in order and applies the node’s effect (for example, creating an object or applying a rotation) to the scene. In certain limited situations, the order in which Maya evaluates nodes can affect the final result.
To view the nodes in order
To change the order of nodes in the Outliner
As you drag, Maya draws a single line to indicate that dropping the node results in reordering, or a double line (above and below the target) to indicate that dropping the node makes it a child of the target node.
To change the order of nodes in the Hypergraph