How to use the Mat4D type to store data in a voxel

[DavidCapek]

Hello, I have just started using openVDB and wanted to use the method described in this solution: https://groups.google.com/g/openvdb-forum/c/kV7bIZbiPNU/m/JmYo6M8ICAAJ .
However, I needed to understand how to implement it fully; what voxel grid type should I use?
And how do you integrate the Mat4D type in the voxel?
Thanks in advance for your help