Levels of Technical difficulty:

1. 1.  Familiar tunes in easiest key. (i.e., Jingle Bells in A Major for Violin, or the First Noel in D Major for any string instrument)  Bowing is almost always "as it comes".  Everything is easy and basic.  No high or low notes in brass; very narrow range for french horn.
2. 2.  A very few accidentals, slurs, or "low 2's" in strings.  Specific bowings, still routine articulations.  A few higher notes in brass, but never in the last measures.
3. 3.  Longer, more complex pieces, with melody in lower parts at times, accompaniment rhythms in all parts at various times, repeats, da capo, routine accidentals, maybe some notes beyond the staff, some string crossings in slurs, 4th finger required sometimes still in easier keys.  Rare extension to high C in I violin.  Bowings may be longer, including "double down and double up" as in the standard Bach Minuets; a few shifts to III position (mostly in I Violin).
   Most of my string pieces are of this difficulty, and most of them were written (and titled) to teach a specific technical aspect, such as "Double-Down Rag", "Long-Slur Samba", or "Lo-2 Blues".
   
   A few pieces in this category may have a II Violin part which is more difficult than I Violin.  This is designed for those players who have mastered I Violin (which is the melody) and need something more to do while the rest of them catch up.  But most of the pieces have II violin parts which are much easier than I violin.  This will be immediately evident in the score which appears on the screen.  Note that the rhythm and bowing of the II violin part rarely differs from I violin.  I do not believe in giving II violinists (or violists) things like back-beats which are so common in music written for professionals--it discourages them, and is in fact quite difficult, compared to the melody.
4. 4.  Much more of the above, with  more accidentals and a little polyphony at times, some mixed meter, inner parts have independent rhythm frequently.  Longer pieces, big dynamic range, III position (in some pieces) more frequent in I violin.  Each part on the level of book II-III, but the texture makes is much harder.  (See the First Symphony for Young String Orchestra, for instance)
5. 5.  Pieces written for accomplished advanced-level players, frequently in unusual combinations of instruments.  Mixed meter, advanced positions, flat keys, double-stops, etc, etc.  The most difficult of these were written for a Juilliard PhD violinist and his associates, and some for myself playing Horn with my music department colleagues at West Chester, PA.