Substudy
A substudy is a smaller investigation carried out within a larger trial, usually on a subset of the participants.
Substudies exist because some measurements are too expensive or too demanding to perform on everyone. Imaging is a common example: a trial of two thousand people might scan a hundred of them.
Findings from a substudy therefore rest on a much smaller sample than the trial that contains it, and the headline number people repeat often traces back to that smaller group rather than to the whole trial.
What it is not
A substudy result is not a finding of the main trial and does not carry the main trial's statistical power. A figure quoted as though it came from a large trial may have come from a hundred people inside it.