Sampled trills are recorded, looped samples, whereas generated trills are produced by manually triggering separate notes.
Because they use fixed sounds, sampled trills typically offer no parameters that allow any kind of variation in the trill interpretation, such as different trill speeds or incorporating grace notes and termination notes into the pattern of trilled notes. By contrast, generated trills can provide greater flexibility but produce a less natural and realistic sound.
If your project contains some trill intervals that can only be generated, you can disable sampled trills and use generated trills throughout to produce a more consistent result.