How does the mybatis parameter replacement work in @SelectProvider

Question

I've in inherited some code that I'm trying to understand and any searching I do to find something on @SelectProvider turns up a whole lot of nothing.

Java DAO

@SelectProvider(type = CategoryDaoSelectProvider.class, method = "findByParentIdAndName")
Category findByParentIdAndName(@Param("parentId") Long parentId, @Param("name") String name);

Select Provider

public class CategoryDaoSelectProvider {
    public static String findByParentIdAndName(Map<String, Object> params) {
        Long parentId = (Long)params.get("parentId");  // WHY IS THIS HERE???

        StringBuffer buffer = new StringBuffer();
        buffer.append("SELECT COUNT(id) FROM Category ");

        if (parentId == null) {
            buffer.append("WHERE parentId IS NULL ");
        } else {
            buffer.append("WHERE parentId = #{parentId} ");
        }

        buffer.append("AND LOWER(name) = LOWER(#{name}) ");

        return buffer.toString();
    }
}

What purpose does the param parentId serve in this code? As far as I can tell it never actually does anything unless somehow magically the #{parentId} is replaced with the value. Is this param just not used in this situation? Where does mybatis actually do the injections into the query?