The Bottom pane contains the 
Proximity pane. If you have PerfectTablePlan Professional Edition it may also contain Custom Field Proximity panes.
If View>User interface>Expert is selected you can hide this pane by clicking on the 
button at top-right of the pane or using View>
Bottom pane or the equivalent Main tool bar button.
You can resize this pane using the splitter between it and the Left pane and the splitter between it and the Top pane.