dummy unit

Ryba-

I am only guessing, but it appears that the "dummy unit" is just that - a non-functional piece of hardware that fills the space where the real units would go if you decided to add them. It might consist of an empty shell with a front piece that covers an otherwise vacant hole.

One reason for such an arrangement would be to maintain the air flow configuration for cooling a rack of equipment. If module spaces were left empty, cooling air would escape, and operational modules could overheat.

In other words, the unit is capable of supporting a number of I/O and CPU modules, but only comes equipped with a smaller quantity. If you decided to expand, you would remove the dummys and replace them with functional modules.

Fred

I think you've got it on the first try.