My understanding of units is fairly simple. It is based on Stanislavski's concept (who I think invented the notion of dividing a script into manageable portions or 'units of action'). Basically a new unit denotes a change in thought or subject. This change can be introduced by the speaker him/herself (e.g. in the first scene of act 1 of The Seagull the son agrees with the brother that he should live in the town then turns to the teacher and the bailiff's daughter to reprimand them for being too early for the play). A new unit can also be introduced by the next speaker (e.g. slightly earlier in the scene the Bailiff's daughter cuts short the teacher's lament about his lack of income with 'the play will be starting soon'). And most obviously a new unit is introduced when a character joins or rejoins the action and therefore interrupts the flow (e.g. in the same scene the workman announcing he's off for a swim). So don't worry so much. Units ain't rocket science, just changes in direction introduced by new thoughts, new ideas, new subjects or new characters.

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