How would you describe the number of shelves of the growing trays in the image below? Four-leveled, four-tier, four-shelf or four-decker? Are all these ways correct and interchangeable?
The picture depicts a seedling stand. They are sometimes also called racks (just like for a computer server.)
When they have wheels (casters), they can be called carts.
Typically, a seedling stand can have various levels called tiers or shelves.
And the shelves are where the trays of seedlings are placed.
In American marketing lingo, there are always attempts to be cute. You can say that a stand has four tiers (meaning levels) and then call it: A four-decker stand. Like a double-decker bus. But this is really marketing language and not a precise technical description.
A four-tiered stand or a stand with four shelves or a stand with four levels (the least descriptive). The tiers or shelves are where the trays or pots are placed.
To my ear, decker is an outlier there, register-wise. decker would not be used in a specifications document, say, as it is something of a colloquialism.
Go and get me a four-decker shelf from the warehouse, and don't dilly-dally.
We would like to place an order for a dozen of your four-tier shelving units.
I would say "four-tier shelf" is the best way to describe it.
"Level" is more reserved for buildings/structures.
"Shelf" by itself conveys a meaning of just a number, but not necessarily that they are stacked in this matter, i.e., "four-shelf" can mean there are four shelves that are arranged side-by-side.
Level could as well be described as high and low, fx. concerning rankings, niveau, terrains, up- or downgrade, strength. Whereas tier describes the level or rank of unique or complete pieces or units, layer describes seperat pieces, sheets or units placed besides or on top of each other as part of a in- or complete unit.