A small group of banks handles the majority of global currency turnover, quoting continuously and warehousing risk rather than simply passing it on. Their pricing is what other institutions benchmark against, and their names appear as liquidity sources in broker disclosures.
For a retail trader the relevance is indirect but real: a broker connected to several tier 1 banks can show tighter and steadier prices than one relying on a single aggregator, particularly during news.
Example: a broker's execution disclosure lists eight liquidity sources including five global banks. In a volatile release, the number quoting shrinks to three and the EUR/USD spread widens from 0.3 to 4 pips for ninety seconds. See spread-widening.
Related: interbank-market, liquidity-provider, liquidity-aggregation, spread-widening