The final version of the documentation for Maya is still under construction. This temporary version is based on the Blender documentation, though its contents is relevant to Maya as well (both variants use the analogical UI).

Custom Target Box

Using the Custom Target Box functionality, you can pack islands to an arbitrary box in the UV space.

In order to use the functionality enable the checkbox in the Custom Target Box subpanel header:

You can specify the box coordinates manually in the subpanel, you can also press the Edit Target Box button in order to enter the Box Edit mode. In the Box Edit mode the add-on renders the box in the UV space so you can modify it with visual feedback.

When in the Box Edit mode, the add-on also displays additional UI in the subpanel, where you have access to additional handy operations for box modification:

After you configure the custom target box and press the Pack button, the packer will pack the islands into the custom box:

Keep in mind that in order for the box to be used, you have to keep the Custom Target Box functionality enabled - as soon as you disable the functionality, the default unit square will be used as the target UV area again (nonetheless the coordinates you configured for the custom box will be saved in the add-on UI so you can easily reuse them later).

Load Box From Preset

All packer settings are stored in preset files also the values of the Custom Target Box.

It is possible to load only the values of this panel from preset file.

This feature can be accessed using the Load Box From Preset icon in the Custom Target Box subpanel: