Traded and Available
The Traded and Available window shows the various amounts of money that have been traded or are available at the various odds for the selection you currently have your mouse over. In addition it also shows the amounts looking to Back and Lay at SP. Once a market is closed for betting the window will just show the traded amounts and the prices at which they were traded.
When rolled up in the Main Info Bar the header displays the current display setting and the number of different prices that there are amounts currently available at or that have been traded. This varies depending on what display setting you have selected as shown below.
By right clicking on the header it brings up the various options for this window.