The blueprint is how you configure what is displayed in the Rerun viewer. It is saved between sessions and is unique to a given application id. The Blueprint includes all Space View configurations, Entity groupings and Entity settings, also known as Data Blueprints.
The reset button resets the entire Blueprint back to its heuristic-chosen default. This includes all settings for Entities, Groups and Space Views.
With this control you can add new Space Views for arbitrary Spaces.
Upon hovering any line in the Blueprint panel, you'll find shorthands for removing and hide/show.
All Entities shown in the blueprint panel refer in fact to their Data Blueprints. I.e. the entity plus the associated blueprint settings. As such, all changes made here are only relevant for the Space View in which they reside.
Whenever Entities are added to a Space View (either manually or automatically), groupings are automatically created. Groups, despite being derived from the Entity Path are independent of logged data. They are meant to improve the handling of large Space Views and allow for hierarchical manipulation of blueprints.
TODO(#1174): Editable & user defined Data Blueprint Groups
To (re-)add an Entity to a Space View, you need first need to select the respective Space View. You then can open a dedicated menu through a button in the the Selection view.