From the description of what the engineers want, a Building Cross-Section looks like a standard Fire Alarm Riser Diagram - with a Plus.
Fire Alarm Riser Diagram
The blueprints of a building typically shows what a building looks like on each floor looking from-the-top. This shows the walls, doors, and windows exactly where they are going to go. This is a diagram to-scale showing details, sometimes down to the placement of the chairs.
The riser diagram shows the building as one looks from-the-side. Because details aren't needed here, this diagram is not-to-scale, and it doesn't even show such details as walls, doors, windows, elevator shafts, stairs, electrical or data rooms, etc.
The riser diagram shows the equipment that will be placed on each floor, it shows the number and types of wires going from floor to floor, it shows each input and output device used on each floor, and even shows the wires along with the type of wire used to connect the devices in each zone to the riser in each zone.
What the standard Fire Alarm Riser Diagram does not show is whether there will be multiple risers located in diverse parts of the building. The standard riser diagram does not have to show anything more about the building than a single line between each zone.
Plus --- Building Cross-Section
The Plus in the Building Cross-Section shows some detail about the building that the standard Fire Alarm Riser diagram doesn't have to show.
It still isn't to-scale; scale can be obtained from the blueprints of the building. It still has the dividing lines between each zone. However it also shows the approximate location of the stairwells, the elevator shafts, the electrical rooms and how they are approximately spaced horizontally with each other, the data rooms, mechanical rooms, maybe even the janitor's closets (if appropriate),etc.
Now, with this diagram, the fire alarm panels will be shown to be in specific rooms, the interconnecting wires will be shown, the location of the devices can be shown in a little more detail, and especially the location and types of risers going between each zone can be shown.
If the risers go up through the data rooms, that will be shown. If the risers return to the main panel through the electrical rooms at the opposite side of the building, that will be shown.
The engineers are looking for more details on this than can be obtained on the standard Fire Alarm Riser Diagram. In essence, the engineers want to know how the system is going to work in the first place, and what is being designed in by the installation company to make sure the system is going to continue to work in degraded conditions (like during a fire).