The Program Backlog 1 is the holding area for upcoming Features, which are intended to address user needs and deliver business benefits for a single Agile Release Train 3 (ART). It also contains the enabler features necessary to build the Architectural Runway 2.
The Program Backlog is created, refined, and prioritized by Product Management 4.
References
1 - Scaled Agile Framework - Program Backlog https://www.scaledagileframework.com/program-and-solution-backlogs/2 - Scaled Agile Framework - Architectural Runway https://www.scaledagileframework.com/architectural-runway/
3 - Scaled Agile Framework - Agile Release Train https://www.scaledagileframework.com/agile-release-train/
4 - CCSQ Agile Playbook - Agile Roles https://qnetconfluence.cms.gov/display/LACE/Agile+Roles