One benefit of holding regular system demos is that execution can be measured across the whole ART. A system demo is a significant event that provides an integrated view of new Features for the most recent Iteration delivered by all the teams in the Agile Release Train (ART). Each demo gives ART stakeholders an objective measure of progress during a Program Increment (PI). The system demo offers the ART a fact-based measure of current, system-level progress within the PI. It's the true measure of ART velocity and progress. Achieving this requires implementing the scalable engineering practices necessary to support Continuous Integration across the ART. By holding regular system demos, the ART can evaluate the quality, functionality, and usability of the solution, as well as identify and resolve any issues, risks, or dependencies. The system demo also provides an opportunity for feedback and learning from the customers, Business Owners, and other stakeholders, which can help the ART to align with the vision and goals, and to adapt to changing needs and expectations. (Must be taken from SAFe 6 Scrum Master resources) Reference:
System Demo - Scaled Agile Framework
SAFe Scrum Master Course Outline

According to the SAFe 6 Scrum Master documentation, the type of work that should fit into one Iteration for one team is a Story. A Story is a short description of a small piece of functionality that provides value to the customer or stakeholder. Stories are derived from Features, which are larger units of value that typically span multiple Iterations. Stories are also decomposed into Tasks, which are the specific activities that the team members perform to implement the Story. Stories are the primary backlog items that the team plans, estimates, and delivers in an Iteration. They are also the basis for defining and committing to the Iteration goals. Stories should be small enough to be completed within one Iteration, and they should meet the definition of ready and the definition of done. (Must be taken from SAFe 6 Scrum Master resources) Reference:
Stories
Features
Tasks
Iteration Planning