In BMW terms, there tends to be an SE designation and a Sport designation.
When the Z4 first appeared in 2003, in the UK all models were designated the SE. The sport designation appeared in 2006 with the face lift variants. Up until this point, all UK Z4s were SE models. Whether it be 2.2i SE, 2.5i SE, 3.0i SE.....
The 2.5Si and 3.0Si refer to the new engines mounted in cars pre 2006, they are however, still SE and Sport trim levels/models.
There you go. Hopefull that answers your question.