Think of stencils as groups of related shapes.
WBS (Work Breakdown Structure) are the current standard organizational structure convention used in schedules. Before WBS was used, Activity Codes (think tags or hashtags) were prominent in the construction industry in the 20th century. The 21st century brought in a hierarchy and thus WBS was born.
Visio uses shapes called containers, that puts a box around other shapes. TimeTable uses containers to group WBS. In the diagram above, there are 2 containers with Phase 1 nested in Construction.
Activities, Tasks, and Milestones are all technically the same 2D shape in Visio.
Links are 1D shapes that represent the relationship between tasks.