Oh! I get to answer an ask about one of my favorite SocShakes things!!
There is an old tradition here of grouping roles together by number when there are enough of them. Usually into Firsts, Seconds, and Thirds*. So any role in the play that is First [X] gets included in the "Firsts" track! For Macbeth specifically, we are including the Witches, the Murderers, and the Spirits. In other plays, you will find citizens, gentlemen, servingmen, messengers, etc.
This is usually a way for us to simplify the process of doublecasting tracks. In some plays, for example this summer's Romeo & Juliet, it makes things harder to spread out the line count evenly**, so we don't always follow it.
*Billy doesn't usually have more than 3 of anything and when he does, there's not usually more than one or two so we don't tend to have "Fourths" as roles.
**I (Jess) have been slowly trying to perfect our process of condensing 30+ roles into tracks for 10 readers. I am not always perfect, however, and if you're like "hey this is a weird choice to double" just let me know and I can look at it for next time!! We all know I forget a random lord now and then.
