What is row_wise_branch which is created only in MT mode?

I’m using Geant4.10.5.1.
When I run the example B4c in multi-thread mode and output to a root file, the columns of ntuple created in B4RunAction.cc are under row_wise_branch as below:

What is row_wise_branch?
It’s no problem for me. But if all columns are just under TTree like those in single-thread mode, it’s better for me.
It is possible?

And I’m implementing a Geant4 application based on B4c.
I added some columns of a vector.
The multi-thread mode also creates columns named “{name}_count” for columns of a vector named “{name}”.
What is this?