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  • Yep. The best way to think of it is "the actor that started the efffort" is the primary actor, and since English dominates in this field, reading left-to-right means putting that actor on the left, and having it interact with stuff on the right, which eventually interacts with other stuff on the right, including the other actors. Calling them primary / secondary isn't an enforced standard, it could also be called initiating / supporting, or first / other, or something else. Commented Aug 5, 2024 at 12:31
  • @EdwinBuck thanks for this feedback. You are fully right: supporting actor is to be preferred. Alistair Cockburn explained in his book p. 59: "We used to call this a secondary actor, but people found the term confusing. More people are now ising supporting actor which is a more natural term". the primary actor is however not the actor initiating the use case (although there might be some overlap), but the actor for which goals the use-case was meant to be. Commented Aug 5, 2024 at 20:15
  • @EdwinBuck Your explanation for the left and the right is indeed an interesting one. I couldn't find the origin of this convention, and putting the primary on the left because it's the first actors (in terms of purpose) because of English reading conventions make fully sense ! Commented Aug 5, 2024 at 20:24
  • fulmanski.pl/zajecia/wdi/zajecia_20142015/materialy/… (1970) which indicated that "default" flow was from left to right, with "right to left" requiring arrows to indicate the non-default direction (page 10). But print setting and typography conventions have longer stated that any information which is to be presented serially, is supposed to be presented in the language's reading order (left to right) which, due to history, in computers was predominately English reading order (left to right). Commented Aug 6, 2024 at 15:50