From the wiki
*Strategic secessions are prohibited. This means creating a new realm, through secession, in order to circumvent recruiting-in-capital-only restriction. Friendly secessions are okay.
*Strategic capital moves are prohibited. This means moving one's capital for the purpose of closer refitting times in a war. Moving a capital to a central region of the realm or for roleplay reasons (historical, realm named based on capital name, removing a rival duke's power base, keeping a rival duke close by to be watched etc.) are acceptable.
If we take the example given to be the only definition of "Strategic" secession it makes things slightly clearer. The rule would appear to be there to stop people trying to get around the capital movement rule by "secession". It does become blured when you are creating a new realm in the lands of your still kicking enemy, but personally I would think that as long as the new realm and the old stayed separate, and there wasn't a mass movement of nobles and lands to the new realm to such that it becomes the dominate force of the two, it isn't "strategic"
