This directory is mainly used for bootstrapping reasons. Suppose a tactic generates a proof term that contains either directly things from Std or custom lemmas/defininitions that make use of things from Std. These lemmas/definitions could not be put into Init for dependency reasons but storing them in Lean directly is also not perfect because we do not want end users to import the compiler. This directory offers a place for such definitions to live, such that the user only has to import Std.Tactic to use such a tactic.

Note that this does not contain meta programs that implement tactics themselves because these would rely on importing things from Lean which cannot done in Std.