The Raven Black wrote:This seems supported by other descriptions on what weapons can have other specific Runes etched on them.
Last I checked, Usage was not the same thing as a Prerequisite or a Requirement. It merely describes how the item in question is properly used.
In the case of runes Usage has a formal meaning.
Each rune can be etched into a specific type of armor or weapon, as indicated in the Usage entry of the rune’s stat block. Explorer’s clothing can have armor runes etched on it even though it’s not armor, but because it’s not in the light, medium, or heavy armor category, it can’t have runes requiring any of those categories.
The first quoted sentence shows that Usage indicates a specific type of weapon that the rune can be etched onto---not that the rune is properly etched onto. The second sentence backs this up by referring to the Usage entry as the rune requiring a category.