The panel preferences can be openen by using the item menu Xfce 面板 ▸ Panel Preferences or by clicking on the Panel Preferences in the the Xfce Settings Manager.
You can select a panel with the combo box in the top of the dialog. Next to this selector there are also two buttons to remove the currently selected panel or add a new empty panel.
The panel selected in the dialog should also show a slection around the panel, so it is clear which panel you are editing.
This tab hold the settings how the panel is positioned on the screen.
Set the panel orientation to horizontal or vertical.
Select the output on which this panel should appear. If the output is not connected, the panel will not be shown.
If you have multiple monitors connected to the computer, this option will appear in the properties dialog. It will set the panel to look at the screen size instead of the monitor size.
Don't show the handles on the panel whcih allow you to move the panel or draw it to snap positions on the bordes of the monitor.
Hide the panel
Change the height (or width if the orientation is vertical) of the panel. The size is in pixels.
Change the width (or height if the orientation is vertical) of the panel. The size is a percentage of the monitor or screen size.
Increase the panel size to fit the contents of the panel. Is work with expanding plugins, like the window buttons plugins.
This tab hold the style settings of the currently selected panel.
This shows the items that are placed on the currently selected panel. In case items are hard to move or edit on the panel using the right-click menu, this tab allows you to move, add, remove and access the preferences of the items on the panel.
Most of the tab is self explaining. With the up and down buttons you can move the currently selected item in the panel.
The add button opens the add new items dialog. With the remove button you can remove the currently selected item van de panel.
The preferences and about buttons are only active if the plugin support these features. It will open the item settings.
Only limition in this dialog, compared to mouse handling, is that you cannot move items between panels and access custom menu items a plugin can add in the item menu.