Ask/Sell orders
Every order book has ask or sell orders containing the list of the open sell order created by the sellers and is organized according to the price levels. The lowest sell price will be ranked at the top and the highest sell price will be ranked at the bottom of the sell order book. This is where all the open sell orders created by sellers are placed. Buyers can select from the prices on the ask or sell orders. The ask or sell orders is organized according to the price levels and will keep reducing as the sell orders get filled from the lowest price levels all the way up to the highest price levels.
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Bid/Buy orders
Also, every exchange order book has bid or buy orders containing the list of the open buy orders or bids created by the buyers and is organized according to the price levels as well. The highest bids will be ranked at the top and the lowest bid will be ranked at the bottom of the buy order book. It is called bid because the buyers are setting their own price and are waiting for sellers to accept the bids and fill their orders. The Bids or Buy orders is where all the open bids or buy orders created by buyers are placed. Sellers can select from the prices on the bids or buy orders and fill the trade. The bids or buy orders is organized according to the price levels and will keep reducing as the buy orders get filled by sellers from the highest price levels all the way up to the lowest price levels.
Decimals
The decimals simply represents the distance between each prices on the sell or buy orders. When there are more decimals, the distance between each prices are very close to each other and when the number of decimal is few, the distance between each prices are far apart from each other. Example, a 0.000001 decimal will mean that the prices are 0.000001 decimals apart from each other. A decimal of 0.01 means that the prices are 0.01 decimals apart from each other. This can come in very handy for users to have a bigger view of the prices. This does not affect the prices, it is only a visual representation, much like grouping the prices.
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I had already heard about using order books to execute buy/sell orders as trading strategies. Thanks @designieplay for sharing this kind of content.