Are we talking about plants, seeds, pots, fertlizers, pesticides, gloves, tools, or something else?

Generally, I would say it means intended for use on plants or in gardens. For example, something like lime might be produced in different grades for different uses. Food grade would require different purity levels than horticultural grade. There are pharmaceutical grades, reagent grades for laboratory use, and so on.

It would help to know to what you are referring. But generally, it means that when a substance has more than one purpose depending upon the manufacturing or processing methods, a horticultural use is its intended purpose.

Diatomaceous earth (DE) is such a product. Hort. grade DE is used to control arthropods in the home and garden.

In the context of soil/sand.

Apparently there is no meaning, or at least none that "Google" could find. You can find all kinds of "Horticultural Grade" products but nothing to tell you what the term means.
Horticulture is the science of cultivating a garden, fruits, vegetables, flowers and grade means an accepted standard, so perhaps you could say "Horticultural Grade" is a product of an acceptable standard for the garden.

George, horticultural grade sand or soil would be processed (screened) to make it suitable for cultural purposes as opposed to, say, construction. When using sand as a propagation medium or as a component in a homemade potting mix, it's important to use horticultural grade sand, which will be coarse, washed to remove salt and dust, and lime free.

Speaking of homemade potting medium, we should also use only that perlite labeled as 'Horticultural Grade'. Same goes for vermiculite and charcoal.

Many of us have purchased horticultural vinegar to use as a herbicide....just another example of the importance of labeling.

George, did your questions get answered?

In Canada there is a reference to grading compost using "horticultural operations" as a use but they don`t necessarily call it "horticultural grade".

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Lloyd