It doesn't loop on part_id or run_id. It will scan all sub directories and loop through all files. It checks each file name (which includes the file path) to see if the number after the second backslash is a number from your run list. If it does the file is loaded, if not it is skipped.
Set vConcatenate = ;
for each FileExtension in 'csv'
for each FoundFile in filelist( Root & '\*.' & FileExtension)
if( match(subfield(filename('$(FoundFile)'),'\',3) & '@', '$(vRunList)') then
LOAD *, '$(FoundFile)' as SourceFile
FROM [$(FoundFile)] (txt, codepage is 1252, embedded labels, delimiter is ',', msq, header is 2 lines);