A padstacks is a special, vertical group of objects. The most typical use cases:
A padstack object (padstack reference in pcb-rnd terminology) is an instance of a padstack prototype. The prototype describes the geometry: one shape per layer type, optionally the hole, and clearances. The padstack object cites a prototype and describes a coordinate (and board side) where it should be placed. The same prototype can be cited by many padstack references. This is how a lot of vias refer to the same prototype, so they all have the same geometry, which can be changed centrally by editing the prototype.
Each board and each subcircuit has its own library of padstack prototypes. User-interface-wide the padstack prototype library behaves very similar to the footprint library.
Select the via tool (leftmost icon from the toolbar):
Move the mouse pointer over the board and click. The geometry of the via padstack palced is determined by the currently selected routing style.
On a padstack reference (e.g. via) already placed: right click on the padstack to get the context menu and select the "Edit geometry/prototype" menu. On the first tab, press the button right to the "prototype" label:
This will pop up the padstack prototype library dialog:
Select a different prototype from the list on the top right and click the "Use selected" button in the bottom right corner. This will change the prototype reference of that one padstack instance the menu was originally onvoked for.
This method should work on any padstack - even the ones sitting in a subcircuit (such as smd pads or thru-hole pins).
TODO: route style