Shell Command

Initially create rectangle either through creating a block command or through sketch.
Then open the shell command from
insert-->offset scale-->shell as shown in below

there are two options in the shell command,
1.Remove faces,then shell
This option is useful when you want to remove the inner material and create a thickness for the out wall as shown in below image

the thickness can be variable with the face,
different faces can select for different thickness

2.Shell all the faces.
In this option out body remains same but inner material removed as shown in below image.

By applying this we cant find the shell applied or not but if you see the section for the block you will find the thickness to the walls.

after section only shell can be visible