http://100fstreet.com/index.php/2011/05/finras-otcbb-service-to-be-renamed-the-non-nms-quotation-service/
National Market System (NMS)
Refers to over-the-counter trading. System of trading OTC stocks under the sponsorship of the OTC. Must meet certain criteria for size, profitability and trading activity.
More comprehensive information is available for NMS stocks than for non-NMS stocks traded OTC (high, low, and last-sale OTC, cumulative volume figures, and OTC and OTC quotations throughout the day).
--- This is due to the fact that OTC must report the actual price and number of OTC in each transaction within 90 seconds verses nonreal-time reporting for non-NMS stocks (thus, last sale prices and minute-to-minute volume updates are not possible).