Legal node names begin with any character from a-z or A-Z and an underscore, followed by a sequence of characters from a-z or A-Z, underscore or numerals.
Previously, Maya accepted illegal names while creating and renaming nodes; for example, ns:0abc. As of Maya 2015, illegal node names are no longer valid. Each time you open, reference, or import a scene with illegal node names, a dialog appears, informing you to update your scene and correct the illegal node names.
To disable the dialog for the current session of Maya, enable the checkbox. To disable the dialog permanently, set the MAYA_ENABLE_LEGACY_INCORRECT_NODE_NAMES environment variable to 1.
To list all nodes with illegal names, execute the command listNodesWithIncorrectNames.