A Subpage is very similar to a Page, except for a few major differences:
Each Subpage can only be added once to any specific Page, but can be used on multiple pages within your project.
This it the name of the Subpage. It must be unique - no Subpages or Pages can have the same name. The name must only contain alphanumeric characters and underscores. No spaces or other special characters are allowed.
This applies the selected theme for the Subpage. The themes can be set via the Theme Manager.
Allows you to adjust and set the dimensions for the Subpage.
Pressing this button will swap the dimension between the width and height.
Pressing this button will resizes the subpage to fit the contents on the right and bottom edges and removes any whitespace on the right and bottom sides of the subpage. It does not affect whitespace on the top and left of the subpages.
Ticking this check box will enable the contents to be clipped (hidden) outside the new size boundaries when the subpage is resized smaller than its contents. If it is unticked, the contents will still render outside the subpage boundaries.
When a Subpage is added to a Page, this is called a Subpage Reference. The Subpage Reference can be moved around a Page and has it's own set of properties separate to that of the actual Subpage that it references.
Name of the subpage. This field is locked and not editable. To edit, go back to the subpage properties.
Join number that will be assigned to the subpage.
Enables the subpage layout to be seen in the main project window view.
X and Y represents the location of the upper left corner of the subpage in terms of its coordinates. W and H represents the width and height of the subpage, both which can only be edited through subpage property.
Ticking this box will enables you to click through to objects below the subpage.
Ticking this box will simply keeps the subpage always on top of others when it is made visible.
This will animate the moving in and out of the subpage from the screen.
To learn how to show and hide a subpage, see this page.