I'm a Jazz Trumpet Player. When playing written music, the lead/conductor sometimes asks for us to mark on the music to remember to hold a note for the full value.
So, consider music is in 4/4 time. A bar starts with an 1/8 note followed by a dotted 1/4 note, followed by a half note rest. The 2nd note is to be held the full value. It is how it is written but sometimes musicians don't play exactly what they should. :-)
So, I've typically written a line over the 2nd note. But someone said, I could write a -2 or -3, and I can't remember which one it is. I think it's -2 to mean hold to the end of the 2nd beat.
I could also read it is -3 means hold until the start of the 3rd beat of the measure.
I'm sure it's simple for someone who knows the answer.