In the OED input files, Risks Attaching basis would be specified by setting AttachmentBasis = 'RA' for the relevant reinsurance contracts.

See linkedin article on Risk Attaching Basis vs Loss Occurring basis.

Risk Attaching basis requires us to take into account whether the underlying policies were incepted in the reinsurance period or not.

I've extended an example in the comments of the above linked in article to illustrate the rule.