The table names are quite easy to guess ( jforum_topics for the topics, jforum_users for user, jforum_posts and jforum_posts_text for opsts and text etc ).
Next, we need to check the posts text. The main job is to see how jive handles bbtags and, if they differ from the way that jforum does, write some java code to port it.
For example, phpbb put some hash in the tags, like [b:s386xda], and jforum does not. So, the migration utility had to remove these hashes before wrting to the database.
Also, date and time are slight as well, so some manipulation were also needed.