I'm away from my Notion computer but I'll do the best I can.... The option I'm thinking of is under strings and wind/brass as I recall, identified as fing. trem. 8th, 16th or 32nd.
The intention is to create an 8th, 16th or 32nd trem alternating between two indicated pitches for the duration indicated, for example a half note. It notates correctly, looks right but doesn't play back correctly. It's mentioned on, I think, p.65 of your 1.5 manual. And your manual adds that it's an expression that will require an update.
It's a super handy device since it saves you the trouble of writing out many, many alternating 32nd notes, but, unfortunately it just doesn't playback.
A fingered tremolando (or shake) is probably to what you are referring. I think it's slated to arrive with the second expanded strings kit in June?
Yeah, string shakes are coming soon, but the only options are minor third, major third, or perfect fourth, and because they're recorded samples, they won't be times to the tempo, more like trills than fingered tremolando.
I am not a string specialist by any means (more a woodwinds person), but I am pretty sure that a perfect 4th is the limit for a tremolando in standard string repertoire. Shakes aren't 'measured' anyway, so all this information sounds right to me!