A scitt is an school based accredited provider in their own right. This means that they can make a recommendation for QTS without the involvement of anyone else.
Many scitts partner a university who will accredit a pgce academic qualification alongside this.
A school direct is a partnership of schools that provides training alongside an accredited provider. They cannot make a recommendation for QTS directly. They can work with either a university or a scitt to do this. A university is more common to work as most candidates want the pgce academic qualification alongside the QTS recommendation.
School direct groups do not have to administer things like student loans, bursaries, international equivalent qualifications ect as these are the responsibility of the accredited provider, as is the collection of evidence for ofsted so smaller alliances find this a more suitable method of working.