Preview features are functions from a future release of GrandTotal. Please note, that this functionality will stop working in GrandTotal 7 after GrandTotal 8 has been released. GrandTotal 8 will be a paid upgrade.
GrandTotal is usually released every 2 years as a new version. So for new features, the users have to wait until the paid upgrade is released. This way, some of the new functions can be used earlier.
Using it is free until the next major version is released. After this you may purchase the upgrade to the new version in order to continue using the features.
In Settings/Advanced of GrandTotal 7.