HELP ME UNDERSTAND THE MEANING OF THIS ?

[tszpun]

Hello Experts,

I am reading GIS specification and i have stumbled upon to a following picture.
It relates to base frame drawing of the GIS.
Could some body tell me, what does this "shadow" mean in the floor plan of the GIS ?
Is there any specific meaning to it ?

Thanks

 

[davidbeach]

Probably the opening in the bottom where cables or conduits can enter.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[stevenal]

I believe the picture indicated the rectangular sections are removeable.