outsideir35.com
Note: I've recently added remote EMEA contracts as another source of leads.
IR35 is regulation intended to prevent "disguised" employment which is an employer hiring a "contractor" to perform the role of an employee. Disguised employment means the employer and contractor both save money on taxes and aren't subject to employment law. A contract is "outside IR35" if it is considered to be a true contract, not disguised employment, which means the contractor has rights to substitution (send someone in their place) and limited responsibilities (i.e: a manager cannot be a contractor).
* "Outside IR35" -> a real contractor
* "Inside IR35" -> an employee in all but name (and therefore subject to employment taxes etc.)
There's no specific accreditation for IR35, "outside" and "inside" refer to how the contract will be treated by the parties involved. There's an expectation that if a contract is being advertised as "Outside IR35" then the company advertising it has assessed that the contract is a true contract. A company can be penalised heavily if a contract is later determined to have been "inside" IR35 (i.e: disguised employment).
Only from the point of view of taxes. Inside IR35 carries all the same risks of outside but pay more taxes.