This is the rule listing that I found on the pinksheets.com website.
Piggy-Back Rule
A "piggyback qualified" security is one that meets the frequency-of-quotation requirement described in SEC Rule 15c2-11(f)(3). The frequency-of-quotation test or "piggyback" exception is based on whether a broker/dealer has itself published quotations in the security in the applicable interdealer quotation system on at least 12 business days during the preceding 30 calendar days, with not more than four consecutive business days without quotations. Once this criteria has been satisfied, authorized participants may register on-line in a security. As long as the security remains piggyback qualified, any participant may quote the security without a Form 211 submission.
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