On the addition of two mesh

Hello everyone,
I generated the following model.

It is composed of two meshes, Sh3 and Th3. The code is as follows.
load"msh3"
load"iovtk"
load"medit"
mesh Sh=square(48,12,[200x,50y]);
mesh Th=square(48,12,[100x,20y]);
Th=movemesh(Th,[x+50,y+15]);
mesh3 Sh3=buildlayers(Sh,3,zbound=[0,-1]);
mesh3 Th3=buildlayers(Th,3,zbound=[0,1]);
mesh3 Zh=Sh3+Th3;
//Sh3=buildmesh(mesh3);
medit(“Al”,Zh);

I want to know the meaning of this code

mesh3 Zh=Sh3+Th3; I guess this code glues the Sh3 and Th3 grids together like glue, right?
I want to know the meaning of this code
mesh3 Zh=Sh3+Th3; I guess this code glues the Sh3 and Th3 grids together, right?
In addition, how to define the label of the newly generated grid Zh?
Thank you.