Switch to data selection meaning?
For an email action, there are 2 different input modes for the fields in Send HTML email action.


Direct input mode: You're supplying most of the data (e.g. the HTML body) with different token (e.g. [site:slogan]) inserted in it.
Data selector mode: You're using a widget (see image) that lets you drill into data that the Rules action has available. This is useful for finding contextual data (e.g. retrieving the current user's email when a node is updated).