Type of Change
Contracts published using the Open Contracting Data Standard can be linked to activities published through IATI, at either activity or transaction level.
Based on advice from @TimDavies …
Contracting process identifier - in OCDS this is the ocid (Open Contracting ID) and should be a globally unique identifier
Contract Identifier - a contracting process might result in multiple contracts (for example, a call for partners that then awards 5 partners, each with their own contract). The Contract ID only has to be unique within the scope of the contracting process, so to unique identify a specific contract requires the pairing of these identifiers
A URL could be provided to a OCDS release package that contains the contracting process and contract discussed. However, OCDS does not use URLs as identifiers, and recognises that (a) some publishers provide multiple processes at a particular URI (e.g. when they only publish bulk data). and (b) some publishers struggle to maintain stable URIs - so we tend to think of URIs as helpful for users, but no guarantee of discovering information.
This was discussed in a TAG session on contracting but was mistakenly overlooked on the Standards Day agenda