Pending Bets
The Pending Bets window is only shown when there are pending bets that have not met their condition yet. When rolled up in the Main Info Bar the header displays the number of pending bets.

When expanded the window displays all of the pending bets.

Menu Options
By right clicking on the header it brings up the various options for this window.

Cancel All Bets - cancels all pending bets
On Removed Runner - Defines behaviour when a horse is withdrawn from the market.
Do Nothing - Pending bets are retained.
Cancel All Bets - Pending bets for the market are cancelled.