Official comment

When you hover over that setting it shows this:
The Full 3D processing is only limited to 5 gigapixels worth of imagery since it is intended for working with a single structure.
This page shows that Full 3D processing is only available using Pay-as-you-go points.
https://www.mapsmadeeasy.com/pricing
All that being said. You probably don't actually even want the Full 3D processing. The standard 2.5D processing that is done by default is more than enough for 99% of jobs that go through the system.