either "void" or "not void" as defined by editor.isVoid
Block vs. Inline
A "block" element can only be siblings with other "block" elements. An "inline" node can be siblings with Text nodes or other "inline" elements.
Void vs Not Void
In a not "void" element, Slate handles the rendering of its children (e.g. in a paragraph where the Text and Inline children are rendered by Slate). In a "void" element, the children are rendered by the Element's render code.