sourcedata folder is optional. It contains a copy of all the raw, unedited data collected from the participant.
'BIDS in its current form is designed to harmonize and describe raw (unprocessed or minimally processed due to file format conversion) data... source data... is defined as data before harmonization and/or file format conversion (for example E-Prime event logs or DICOM files).'
It holds a folder for each subject, and, within these, one or more session folders (for multi-session datasets).
As the data here depends on the lab and study, nearly all data is optional. The below are those we collect on the day of testing for our clinical fMRI cases.
Contents: Lab-specific (examples for Benjamin lab)
|README||[file] [plaint text] [Optional]. This is user-defined. It contains any information researchers consider necessary. We include here data on issues with data collection.|
|sub-[x]/ses-[y]/||[folder] [Required]. One such folder is required for each participant. This holds the raw, unedited data acquired for each participant / session.|
Note: Full details available at BIDS website; the full specification is definitive.