All IPSO's national newspaper publishers are members of the compulsory scheme. That means that if you want to make a claim they must agree to arbitration. Some other publishers participate in the voluntary scheme, which means you can request to arbitrate but the publisher is not obliged to do so.
Your claim may be struck out at any point during the above process, but usually at the earliest possible stage. You will be given notice of this and a chance to respond. Your claim may be struck out if the arbitrator determines:
Following a strikeout, the arbitrator will require you to reimburse the publisher's fees and may require you to reimburse their legal costs (up to the cost cap).