Depth of book
An order book is the full set of bids and asks waiting to trade, ordered by price. Level 2 exposes this set: below the best bid are further bids at lower prices, and above the best ask are further asks at higher prices, each with its own size. Observing several levels at once indicates how much buying and selling interest is queued around the current price.
Level 2 versus Level 1
Level 1 is the top of the book: best bid, best ask, and last trade. Level 2 is everything beneath it. Moving from one to the other is the difference between knowing the current price and observing the structure of supply and demand behind it. This additional context is informative but involves more data and is relevant only to certain forms of analysis.
What the depth reveals
Order book depth is an indication of liquidity. A book with substantial size across many levels suggests a market that can absorb larger orders with limited price impact, while a thin book suggests the opposite. Depth also indicates how far a large order might move the price as it consumes successive levels. This is descriptive information about market structure rather than a prediction of future prices.
When the additional detail is warranted
Level 2 is most useful for execution-sensitive work, such as placing larger orders, developing market-making or microstructure strategies, or studying liquidity in detail. For general charting, valuation, and alerts it is typically more than necessary, and it involves a greater volume of data. Most applications operate on Level 1 and use Level 2 only where the depth materially affects a decision.
Depth and aggregated pricing on SiftingIO
SiftingIO concentrates on clean top-of-book pricing and a single aggregated fair price across stocks, forex, crypto, and commodities, reconciled from multiple independent sources under one schema. Where full order book depth is required, the data methodology describes what SiftingIO currently publishes and how the aggregated price is constructed, and additional datasets can be requested.