tl;dr that picture is very poorly labeled and contains at least two errors.
This is probably the original that has been copied over the years, from the 1985 Shuttle News Reference (online version does not include picture).
An "avionics bay" is a volume filled with computers and other 'black boxes'. There were three of them in the Orbiter's crew compartment as shown in this drawing from the SCOM. The equipment in them was cooled by either forced-air cooling or by coldplates serviced by the Orbiter's water cooling loops. (There were also three unpressurized aft avionics bays in the Orbiter aft fuselage; the equipment in them was cooled by coldplates served by the Freon cooling loops.)
The "instrumentation rack" in your drawing appears to refer to Av Bay 3A.
Here is a picture I took of equipment in one of the avionics bays in an Orbiter when it was in the Orbiter Processing Facility (OPF).
'Aluminum plate' and 'open grid panel' appear to refer to structural elements, although 'open grid panel' is an error. The picture shows it between the flight deck (upper compartment) and mid-deck (lower compartment) but there was no grid there, just two rectangular access holes.
You can see in this picture from the STS-131 mission that there is no open gridwork in the area called out in the illustration. Mission Specialist Naoko Yamazaki is in one of the access holes.
The 'lower deck' shown in the illustration was commonly referred to as the 'Lower Equipment Bay' or the 'ECLSS1 bay', and was quite cramped. The beer-keg-like cylinders in the drawing would seem to be the supply and waste water tanks, which were indeed located in that area.
Here (again from an Orbiter in the OPF) is a picture of those tanks I took in the Lower Equipment Bay.
1 Environmental Control and Life Support System