Level I or II Market Data Subscription Differences...

Market data includes information on completed trades as well as current price and volume availability in various financial markets. 

Traders subscribe to the market data for their markets through their broker, and will choose their market data subscriptions based upon the markets that they are going to be trading. For example, a trader that wants to trade individual stocks will need a market data subscription to the NYSE or NASDAQ stock exchange, or whatever stock exchange the stocks they want to trade are listed on.

A trader that only wants to trade a specific futures contract (or several) will need to request market data for those specific markets from their broker.

Forex brokers typically offer Level 1 data for all their product offers, while some also offer Level II market data for all their offers. With forex brokers, you don't need to subscribe to the data. When you log into your trading platform it should already be available to you.

Level I and Level II Market Data

Market data is usually available is two different subscriptions (depending upon the markets in question), Level I markets data and Level II market data.

Level I market data includes all of the standard trading information for a market, which is the following:

• Bid price: The highest price that a trader is willing to buy an asset at.
• Bid size: The number of shares, forex lots or contracts that are available at the bid price.
• Ask price: The lowest price that a trader is willing to sell an asset at.

• Ask size: The number of shares, forex lots or contracts that are available at the ask price.
• Last price: The price at which the most recent trade was completed.
• Last size: The number of shares, forex lots or contracts that were traded in the most recent trade.

Level II market data includes all of the standard trading information for a market (Level I market data), and some additional trading information:

• Highest bid prices: The highest prices (usually at least several prices are shown) where traders have placed orders to buy. This means you see the the current bid, and bids currently below it. Actively traded stocks typically have bids every $0.01 below the current bid, and in actively traded futures, there is typically a bid each tick below the current bid. Gaps between the current bid and next bid typically means the stock or contract has a larger bid/ask spread and less volume.
• Bid sizes: The number of shares, forex lots or futures contracts available at each of the bid prices.
• Lowest ask prices: The lowest prices (usually at least several prices are shown) where traders have placed orders to sell. This means you see the the current ask, and asks currently above it. Actively traded stocks typically have asks (offers) every $0.01 above the current offer, and in actively traded futures, there is typically an offer each tick above the current offer. Gaps between the current ask and next ask typically means the stock or contract has a larger bid/ask spread and less volume..

• Ask sizes: The number of shares, forex lots or futures contracts available at each of the ask prices.

Level I market data provides all of the trading information that is required to display a graphical chart of a market, and the time and sales of a market. Level II market data provides additional trading information which is most often used by day traders in attempt to make short-term predictions on the direction of the price.

The attached chart shows bid and ask prices at multiple price levels (with the shares available at each price) in the SPDR S&P 500 ETF (SPY). 

Level I or Level II Market Data?

Many new traders do not know which level of market data they will need, and therefore subscribe to all of the possible market data. Since you pay a fee for each market's data, every month, subscribing to unnecessary data results in unnecessary trading costs. 

Most traders only require Level I market data because Level I market data provides all of the trading information that is needed to display the price charts that they will use to perform analysis and make trading decisions. For many traders, watching the constant flurry of changing bids and ask prices on the Level II will result in information over-load, which could actually have a detrimental effect as opposed to a positive one.

Level II market data might be required for some trading strategies that attempt to isolate strong buyers or sellers in the Level II data, and then piggybacking on the direction that buyer/seller will push the price in the short-term. This is typically a scalping strategy, where traders take advantage of short-term patterns they see in the bidding/offering activities of other traders in a particular market. 

The Correct Way to Choose Market Data

If you are a new trader, then you only need level I market data for the specific markets you want to trade. Opt to keep your costs as low as possible at the beginner of your journey. If want to trade more markets later on, or try using Level II data, you can always tell your broker to add it on later.