FOP and Subscript offset

In the latest DocBook XSL (Stayton), p. 209 states that subscript 
offset in FOP does not properly work using 'baseline-shift'.
It states that 'a different property must be used for that processor'.

Could someone please comment on this 'different property'?

Cheers,        Tristan
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Tristan J. Fiedler
Postdoctoral Fellow - Stein Lab
Cold Spring Harbor Laboratory