That is how I imagine UST. If there is intent at romantic/sexual subtext or if there are character thoughts that reflect the same.

Yet, at the same time, I wonder how many fics might be labeled UST but might not really be filled with UST because either the author felt pressured to use the label or might have a different definition of what UST is.

You know how hypersensitive the fandom, any fandom, can be.