confusion about labels in overview -> specification

[cwlucas41]

in overview -> specification the following bullet point is confusing to me coming in with zero knowledge:

Each step of the algorithm belongs to a separate label. The labels determine what happens atomically and what can be interrupted by another process. That way we can represent race conditions.

each of the other bullet points reference text from the spec for what is being explained (like People and acct) but this one does not. After following the link, I assume Check:, Withdraw:, and Deposit: are the labels, but I'm not 100% sure and just have to continue reading.

[cwlucas41]

update - reading linearly it's not until here that I'm confident in what labels are