Since you are new, I will review some of the basics. When you create a Visio
drawing, you can create shapes from scratch or drag them from a stencil.
Shapes on a stencil are actually master shapes. When you drag a master shape
from a stencil, in addition to placing an instance of the shape on the page,
it also creates a copy of the master within the document in an area called
the document stencil.
If you edit it a master shape on the document the changes can be replicated
to all the instances of that shape within the document.
To get to the Document Stencil.
View --> Toolbars --> Stencils
The document stencil is the second one and looks like a white piece of paper
with the bottom left corner turned up.
John... Visio MVP