OTC Markets Group operates the largest US over-the-counter equity marketplace and classifies every traded security into tiers based on the issuer's disclosure quality and listing eligibility. OTCQX is the highest tier and requires audited financials, current disclosure and SEC reporting (for US companies) or compliance with home-country regulators. Many large foreign companies trade on OTCQX as ADRs (Nestle, Roche, Tencent).
OTCQB is the venture tier — early-stage and developing US and international companies with current SEC or alternative reporting, minimum bid price of $0.01, and an annual certification process. Pink Current means the issuer files some form of disclosure but does not meet OTCQX/OTCQB standards. Pink Limited (also called 'Limited Information') means very limited or stale disclosure. Grey market securities are not actively quoted by any market maker and trade rarely, with no public price discovery.
Broker access varies dramatically by tier. Mainstream brokers (Schwab, Fidelity, E*TRADE) generally support OTCQX and OTCQB but restrict or block Pink Limited and Grey market trading. Day-trader-specialist brokers (TradeStation, Cobra, CenterPoint) and some international brokers (Interactive Brokers) provide broader access. The buystock.net broker matrix breaks down per-tier access explicitly.