In the podcast episode Joe Rogan Experience #1425 - Garrett Reisman they were talking about what’s like to work for Elon Musk and then Garrett mentioned this:
Tesla is kind of like plan A save this planet and SpaceX is kind of plan B.. - Garrett Reisman
With plan b he means, if this planet will be destroyed by whatever cause, SpaceX has the goal to make humanity live multiplanetary so that humanity will continue to survive.
See the part at 9:35 here: