Simcha Fischer has all the details:
When I spotted this app, I was delighted to see that it’s extremely similar to my Creighton chart. It’s designed for the Billings Ovulation Method, which tracks a woman’s cervical fluid to establish times of fertility.(NB: I’m using a Billings app for a Creighton method of charting, and have gone off the grid with my teacher. I don’t endorse or recommend this behavior! For more information about Creighton, which is about more than achieving or avoiding pregnancy, see their website to get the whole scoop.)
Read more here.