[1] https://en.wikipedia.org/wiki/Regulation_D_(SEC)#Rule_506
I have seen this done when someone comes along who would really like to invest in your privately held company but the company doesn't want to go through the whole process of evaluation and term negotiations of a new funding round. If your current investors are ok with it, you re-open the previous round (same price per share so only your post money valuation changes) same terms and same rights. If everyone signs off then you collect the money, issue the additional shares and carry on.
At the time (July of this year) they were valued at $21B so $100M is about .5% of the company. I'd probably take that deal if I had $100M to spare and Elon would let me put it into SpaceX :-)
Also, does this mean that SpaceX could raise funds indefinitely without going public as long as they come from the same source?