When you have downloaded a work area to the tablet, you are allowed some leeway in plotting and inspecting assets over the boundary of the area selected.
However, if you stray too far away from the boundary you will no longer be able to add new assets, and you will receive a message telling you that you are more than 50m outside that boundary. it is not possible to add assets further than this distance from the boundary of your current work area.
In Grass SMART, you will only see this error when working in a work area, not in a work package. You can simply download the neighbouring area to continue working and plot the new assets.