7.2.4 WS:  The Wyckoff sequence 
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This is an alphabetic list of the Wyckoff symbols (letters) of
the occupied special positions. International Tables for Crystallography Vol. A lists the Wyckoff letters for all
special position, that is, all sites having a crystallographically distinct site symmetry.  Each letter is followed by a number indicating the number of symmetry-independent atoms occupying sites of that kind (the
number 1 is omitted), e.g, 'a1 d1 i6' which is written as 'adi6'.  The enumeration list contains all the letters of the alphabet plus '&' for 'alpha' found in space group 47.  The letters are listed in alphabetic order but before determining the Wyckoff sequence, the structure must be normalized according to the algorithm described by Parthé & Gelato (1984, 1985).  This algorithm is based on the use of the standard space group settings given in International Tables A with the lattice parameters that are not determined by symmetry chosen according to a particular set of rules.  The atomic coordinates are then transformed to ensure a consistent choice of Wyckoff letter in the case where several Wyckoff letters refer to sites having the same symmetry*.  

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The Parthé-Gelato (1984,1985) algorithm was selected because it is, to our knowledge, the only one that normalizes the whole structure including the atomic coordinates.  Other algorithms normalize only the space group setting and the unit cell basis, which by themselves are not sufficient to ensure a unique Wyckoff sequence.  The Parthé-Gelato algorithm has been tested in several applications and has been shown to be robust and furthermore it has been implemented in the program STRUCTURE TIDY which includes the Wyckoff sequence as part of its output (Gelato & Parthé 1987).  The conventions used in the Parthé-Gelato algorithm differ from those adopted elsewhere in crystallography (see the discussion in Parthé & Gelato ,1984) since they are optimized to produce a robust structure normalization, while other conventions are designed for different purposes, e.g., to emphasize the physical or chemical properties of the crystal or to provide a cell descriptor that is easy to search.  The space group setting and unit cell basis used in the Parthé-Gelato algorithm, while essential for generating a unique Wyckoff sequence, are discarded after use and do not appear explicitly in the Crystallographic Phase Identifier.  They therefore do not conflict with the different space group and unit cell conventions adopted by the databases using the Identifier. 
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