Self-help
Quick definition
Self-help is one of the major exceptions to Rule 611 of Reg NMS that allows market participants to disregard the protected quotations of other venues that are experiencing systems problems.
What is Self-help?
Self-help applies to both US equities and options trading, which are subject to Reg NMS.
Some exchanges, like IEX and MIAX, actively post system status messages when they declare or revoke self-help against other trading centers. See for example:
References
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SEC Division of Market Regulation. (2008) "Responses to Frequently Asked Questions Concerning Rule 611 and Rule 610 of Regulation NMS."
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SEC Division of Trading and Markets. (2015) "Re: Rule 611 of Regulation NMS."