Definition
A CSA National Instrument is a rule drafted by the Canadian Securities Administrators and adopted by each of the 13 provincial and territorial securities regulators through their own rule-making process, giving the NI legal effect in each jurisdiction. Once adopted, the NI becomes provincial or territorial law; for example, NI 31-103 is simultaneously OSC Rule 31-103, BCSC Rule 31-103, ASC Rule 31-103, and so on. Companion Policies (CPs) are explanatory documents issued alongside NIs to interpret their provisions, but CPs are not legally binding. National Instruments take precedence over CIRO rules when there is a conflict, because CIRO rules must be consistent with the NIs under CIRO's recognition order. The NI numbering convention follows a three-part scheme: the first two digits indicate the topic area (21 = marketplaces, 31 = registrants, 45 = distributions, 81 = investment funds), followed by a sequence number.
Source
CSA website (securities-administrators.ca); National Policy 11-202 (passport procedures)