It should cover issues such as the position of people and firms that live and work in the EU, thanks to the current freedom of movement, but will find themselves on the other side of what will again become an international border in all respects in 2019: this applies to British citizens in Europe and to European ones in the UK.
This agreement might establish transitional rules and temporary or permanent, conditional or  unconditional, recognition of status, on people’s right to stay where they are and continue with their current activities. Another important issue, which will presumably be covered by the withdrawal agreement, is the UK’s financial obligations towards the EU as a member state.