What is the difference between the order processing types DD, NDD, STP?

1. DD (Dealing Desk) — Forex brokers, called market makers, literally “create a market” for traders, as they can establish their own levels of quotations.
2. NDD (Non Dealing Desk) — orders are executed directly, without a dealer. An NDD trader connects directly with the financial market. The advantages of the NDD platforms are more stable quotes and the ability to trade without restrictions in the circumstances of strong volatility of the market.
3. STP (Straight Through Processing) — STP orders are sent directly to liquidity providers. An STP system can be connected to one or more liquidity providers. The more liquidity providers, the greater liquidity in the system and, therefore, the best performance for clients — spread and execution.