More specifically, it appears this is set up as an Option to Buy, according to the website: https://www.unison.com/read_more/1U8IgX7as5VFPbNLtvehNd

 I would think of it the same as a Right of First Refusal, as far as title is concerned.

It will be interesting to see how traditional lenders handle these documents. There is a provision to 'buy out' the agreement, which would otherwise be allowed to run 30 years or until sale. Lenders may require the buy out before offering a new mortgage, which would cause the document to be treated as a lien, from a title perspective.