Each dataset supplied by UK Biobank is pseudonymised according to a unique Application specific scheme. This means that data supplied to researchers under one Application ID cannot readily be related to data supplied to other researchers using a different Application ID.
This security feature becomes problematic when researchers wish to retrieve an Archive dataset deposited with UK Biobank by a previous project. Bridging is the procedure whereby one set of researchers is supplied with a file which links their psuedonymous IDs to those of a previous project. Bridging files are (after appoval) downloaded using the ukblink utility.