There are a near limitless number of ways that we can control our geometry using the geometry nodes system including the ability to reassign, rotate, and redistribute the various points of our mesh geometry. With many tools we can take this control to greater heights by using vertex groups.
A vertex group becomes a custom attribute in geometry nodes which when created can be found in the points domain of the spreadsheet. Connecting a parameter to a group input node will then allow you to call that vertex group by pressing the cross button in the modifier. It can then be used to control where a node affects a model, like isolating the top face of a cube.
There are other ways that we can use vertex groups in our node set ups as well, including using said groups to distribute instanced geometry or defining the areas of the model that are extruded.