The goal of Sprint Planning is for the team to make a commitment to the Product Owner as to what they’ll bedelivering in the upcoming Sprint. The meeting is timeboxed to 4 hours (for a 2 week sprint, or double that for a 4 week sprint) and consists of two parts. The first half of the meeting is when the Team asks questions of the Product Owner to help clarify the PBIs in question. If these items still need to be estimated, this is also the time that’s done. In the second half of planning, the team breaks down the PBIs to which they are committing into tasks. During this second half, it’s possible the team will need additional clarification from the PO, though it’s less likely at this stage, so the PO doesn’t necessarily need to be present for this half, but he should be accessible just in case.
As I said, the goal of this meeting is for the team to commit to what they’ll be delivering to the PO by the end of the Sprint. Traditionally, this commitment is made at the end of the first half of Sprint Planning, before the team has broken PBIs down into tasks. Some teams however are more comfortable making a final commitment after they’ve created tasks in the second half. That’s fine if it works better for your team, though I’d encourage you to try making commitments in the first half. With practice, teams do quite well basing their commitments on the estimates (usually in story points) without having to consider the tasks.”