What may be included in the Sprint Backlog?

The Sprint Backlog is the set of Product Backlog items selected for the Sprint, plus a plan for delivering the product Increment and realizing the Sprint Goal1. The Sprint Backlog is a forecast by the Developers about what functionality will be in the next Increment and the work needed to deliver that functionality into a “Done” Increment1. The Sprint Backlog makes visible all the work that the Developers identify as necessary to meet the Sprint Goal1. The Sprint Backlog can include any items that the Developers deem necessary to achieve the Sprint Goal, such as user stories, tasks, use cases, tests, or others. The Sprint Backlog is a plan with enough detail that changes in progress can be understood in the Daily Scrum1.

References:

The Scrum Guide, section 3.3. Sprint Backlog

Professional Scrum Product Owner I (PSPO I) Learning Path, section 2.2. The Product Owner and the Sprint

Professional Scrum Product Owner I (PSPO I) Sample Questions, question 11