[1] Crystallography tables give the Wyckoff positions for different space groups.
His 1922 book, The Analytical Expression of the Results of the Theory of Space Groups,[3] contained tables with the positional coordinates, both general and special, permitted by the symmetry elements.
This book was the forerunner of International Tables for X-ray Crystallography, which first appeared in 1935.
The letters are assigned in alphabetical order with earlier letters indicating positions with fewer equivalent positions, or in other words with larger site symmetry groups.
For a particular space group, one can check the Wyckoff positions using International Tables of Crystallography.