What are these measure numbers called. I could have just answered my own question there. See highlighted below. Who decides these? Is it the composer? You see I have noticed sometimes they are in spaces of five bars apart, other times a little longer.
Also, is this perhaps in rehearsal the conductor can say “From 39” or similar?
Stravinsky, Symphony in C
These are normally called 'rehearsal numbers' or 'rehearsal figures'. You're correct: the conductor will say "from number/figure 184" or just "from 184" or even "184!". Often rehearsal letters are used instead of numbers (rehearsal numbers and letters are referred to as 'rehearsal marks'), and sometimes every measure is numbered individually in a small font. The spacing can be as short as every four or five measures but sometimes they are spaced irregularly and only placed at the beginning of musical sections.
A composer might put these in themself, but if they don't the publisher will. This occasionally leads to problems if the conductor and the orchestra are using different editions with different rehearsal number systems. A publisher might have the orchestra parts for an old work computer-typeset and change the numbering system and then the conductor turns up with the score they have been using for forty years.
Rehearsal marks without doubt, for bar numbers their difference is too small.
See also this question; it is a bit unusual, that the rehearsal marks follow so close to each other, since 3rd bar after xxx is also frequently used.
I have not yet encountered three-digit ones. Rehearsal marks may also use single or at worst double letters and be surrounded by a circle instead of a box.
That aren't measure numbers but rehearsal marks. They can signal beginning of solos, difficult sections, or any useful starting place to rehearse. To allow some freedom and for quicker finding of marks, frequent placement is recommended.
The person conducting rehearsal can indeed say: "let's start again from 39" and the players easily find where that is. It's much more practical than counting measures. Rehearsal letters instead of numbers are common.
The measure numbers in a musical score are typically added by the editor or engraver who prepares the printed or digital score for publication. The music composer does not usually specify measure numbers in their original manuscript, although they may have used other methods to indicate different sections or points in the music.
A measure number helps performers and conductors navigate the score and communicate about specific points in the music, usually being placed at regular intervals, such as every five or ten measures, may vary depending on the length and complexity of the piece.
During rehearsals or performances, conductors or other performers may refer to specific measure numbers as a way of indicating where to start playing or to focus on a particular section of the music. For example, a conductor might say "Let's start at measure 39" to indicate where to begin rehearsing a certain passage.
