how to write in classical Latin: "beginning of the act" and "ending of the act"?

By "act" do you mean an action or a division of a play?

in this case "beginning of the act" means the opening music intro at the beginning of action and the "ending of the act" means music that sounds at the end of this action. several actions and each begins and ends with these two musical motifs. two music motifs divide the play on parts "acts" and the actions every time is between these two musical motifs beginning and ending, which is repeated for each beginning and ending of each new action "act". yes it can be called a division of play but at the same time it is the entry and completion of each action of play

after some research I have here a few options with the translation, it would be useful to hear your opinion

exordium actus - beginning of the act
initium actus - beginning of the act
incipit actus - the act begins
explicit actus - the act ends
finis actus - end of the act

I would pair
initium actus . . . exitus actus
-or-
principium actus . . . finis actus

thank you!

can you give me a direct translation to understand the difference between these versions:

exitus actus - ...
principium actus - ...

Exitus and initium are derived from the words for going in and going out,
rather appropriate fro a play.

Principium and finis are often paired, most famously in the Apocalyse.
Ego sum Alpha et Omega principium et finis
I am the Alpha and Omega, the beginning and end