R help - Mar 2010 - a question about Joint Model(JM)

Hell sir:
I'm a user of JM and have a quesion.
As to "JM: An R Package for the Joint Modelling of Longitudinal and
Time-to-Event Data", on page14:
fitSURV <- coxph(Surv(Time, death) ~ drug, data = aids.id, x = TRUE)
 
I don't understand why use the dataset "aids.id", but
not"aids" as follow:
fitSURV <- coxph(Surv(start, stop, event) ~ drug, data = aids);
 
Thanks a lot for your help!
 
My best.
 
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