In the free BIMx App you can purchase the BIMx Docs functionality with the In-App purchase options. There are two kinds of In-App purchase options in the App Store: consumable and non-consumable.
The “License for all your Projects” option is a non-consumable license type. It can be used on four other devices and it can be restored on the same machine (if the App was deleted) with the same Apple ID that was used at the original purchase.
To restore your purchase follow these steps:
Three options appear, scroll down to the last one and choose the “Restore Purchase” option.
Sign in with the Apple ID you used for the original purchase. After this your purchase will be restored.