In Blender 2.8, we want to make it easier to set values for multiple objects or bones at once. In version 2,79 you can do this, but you have to right-click on a value after you've changed it at pick Copy to Selected from the menu. This is both slow and unnecessary. You can also hold Alt to affect all selected items, but this is not discoverable, and clashes with the popular Emulate 3 Button Mouse option, which takes away the ability to use the Alt key here.
Instead we want to make it so you can simply type a value, hit return, and that value then automatically gets propagated to all the relevant objects or bones.
//Note: For technical reasons, we cannot make this work for nodes, because it's not easy to figure out how to find the correct pair mapping. So we need a way for the user to know which items can work and which items cannot. We also cannot do real-time compares of 3000 selected objects to display special indicators if values are the same or they differ. So we have to work around these limitations.//
This is how we've decided to solve this:
If the user has more than one object or bone selected, and then goes to edit a value, we display a 'selected' outline around the property, to indicate that this will affect all selected items. The user simply types a value and hints return.
Additionally, we want to make it easy to type in // relative // values too. We allow this by using =, so the user can type '= -3' to subtract 3 from the value on all selected objects or bones, like so:
To edit the value of only the active item, even if multiple ones are selected, we can invert the Alt-key behaviour, so holding Alt will only edit the value on the active item.