Considerations for Agile/Adaptive environments
Typically, in Agile environments, contracts are used to extend the team and have a greater need for flexibility since the scope of work changes and is non-predicative. Due to this need, there may be a master service-level agreement (SLA), or allowances for scope changes or adapting the project needs without having to renegotiate or change the contract. SLAs also allow for a shared risk of responsibility and more collaborative working relationships.
All of these factors would be determined as you plan for procurement iteratively throughout a project, depending on the procurement needs.