This refers to the full Public Records Office reference for the page of the census on which this person appears. It is in the format:
[PIECE] / [BOOK] / (~) F[FOLIO NUMBER]
eg: HO1070018/2/~F5
'~' Denotes the folio number has been calculated using the PDF page number within the file. Whilst this is normally accurate, where pages may be missing from the original documents a folio number may appear to be a page or so out of sync.
'?' instead of the folio number means we have been unable to calculate an accurate folio number.