I don't know what its official name is but I would call it 'variable geometry'. Basically, every shape in Visio has a 'Shapesheet' that controls the attributes and behaviour of that shape. To make this far more complicated, shapes can include sub-shapes so that you may need to drill into a 'group' to see the shapesheet of the lower level or child shapes. The attributes of a parent shape can be derived from one of its children (subshape) or vice versa.

In the case of that particular shape, looking at its shapesheet doesn't reveal much of interest, however if you ungroup it you can select the subshape and view its shapesheet where you can discover a series of geometries which have a Visible property that is linked to the parent's Shape Data.

You are branching into a brave new world which can get very complicated in a hurry. You can find a bunch of resources online. Here is one link to a site that can be useful to get you started.
The Hidden World of Visio Shapes – Visio Guy

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Andrew Lockton
Chrysalis Design, Melbourne Australia