During the sprint planning the SM asks the team members their capacity in hrs/days in the sprint so the team can understand how much a team member can take. If the sprint is two weeks a member can be the first week on vacation therefore his capacity is 50% or 40 hrs in that sprint.
I also use MS TFS which has this feature to help the team understand how much they can plan for in the current sprint. It quantifies it in hours or days.
