In my copy of Jacques Ibert's Concertino da Camera, I have this curious marking:
In the first bar after 23, there seems to be a tremolo from a C# to a B. This shows up multiple times—why isn't this just written as a trill? Is there more to this since it's written as a tremolo?
The distinction here is presumably one between a measured tremolo and a trill open to your interpretation.
With the trill, you can determine how quickly you play it (16ths, 32nds, triplets, or something else?), how consistently you play it (will it begin at the same speed that it ends, or that it is in the middle?), etc.
But the measured tremolo here must be comprised of 32nd notes because of the three bars between the pitches. One bar would mean eighth notes, two bars would mean 16th notes, and three bars means 32nd notes.
It is probably written this way because a trill generally means to trill to the note a major/minor second above (as can be seen in the bottom line of the image), but here Ibert wants a trill between C# and the second below. There are alternatives to this notation (like writing a trill on B with with a preceding C# acciaccatura, or on C# but with a small-notehead B in brackets after the main note) but the notation with tremolo is often used for this situation where the trill is in the reverse direction, or where the interval is greater than a second.
A three-beam tremolo generally means a fast, unmeasured tremolo (i.e. like a trill) unless the tempo is slow, or unless the composer gives any specific direction one way or another (sometimes you see 'trillo' or 'non trem' or similar at these places as clarification).
One possible reason is that the composer specifically wants the note that falls on the beat to be the higher of the pitches. Trills (unless indicated otherwise e.g. by a preceding acciaccatura) begin on the lower pitch.
