boundary condition between mesh blocks
 

What's the use of boundary condition in the intersection of two mesh blocks?
If the fluid can pass through it freely, which BC type should I choose?

You should use symmetry BC (this is the default BC).

FLOW-3D resets the boundary condition between blocks automatically to "inter-block" type. You can't set this yourself, FLOW-3D does it for you. Using 'symmetry' prevents some warning messages about resetting the condition.

I wonder if you could tell me what the meaning of BC Grid Overlay

thank you all. Maybe I haven't read the manual carefully enough

Grid Overlay boundaries are only used in Restart simulations. They interpolate velocity from an existing simulation results. You can use them to change the mesh but still continue from the old simulation results.