A rule that forbids naming the fiber
On this page10
The sentence is short and it is a prohibition
No fiber present in the amount of less than 5 percent of the total fiber weight shall be designated by its generic name or fiber trademark in disclosing the constituent fibers in required information.
And it supplies the substitute
It shall be designated as other fiber. Where several such fibers are present they are designated in the aggregate as other fibers, so two suppressed fibers become one line.
Which is unlike every other spine in this library
A net weight, an ingredient list, a hazard class, a volume statement, a care instruction. All of them are rules about something that must be said. This is a rule about something that may not be said.
The suppressed fact is not false
A fiber at four percent is genuinely in the product. Naming it would be accurate. The rule prohibits the accurate name anyway, in favor of a category.
There is a reason for it and the reason is comparability
A content statement is read as a ranking. Allowing trace fibers to be named by weight would let a product foreground a desirable fiber present in a quantity that does nothing, and the floor removes that move by making the small entries indistinguishable.
And the exception proves that is the purpose
The proviso allows a fiber to be named if it has a clearly established and definite functional significance when present in the amount contained in the product. The example given is 96 percent Acetate, 4 percent Spandex.
So the test is not whether it is there, it is whether it does anything
Four percent spandex changes how a garment behaves. Four percent of a decorative fiber generally does not, in that sense, and the rule treats the two differently on that ground rather than on quantity.
The threshold is by weight, which matters for what follows
Total fiber weight. Not area, not length, not visible prominence. A fiber can be the most noticeable thing about a garment and still be under five percent of its weight.
Which is exactly the situation embroidery is in
Stitching that covers a visible patch of a shirt front weighs very little against the shirt. The next section is the part of the rule that deals with that, and it measures a different quantity entirely.