How to decide these tips? Find the cells along a certain path?

[chengkeren]

Hi,
Thank you for providing this package, really love it.
May I ask something?
I found a very interesting thing, I run destiny many times using different data, why there is always 3 tips?

And I want to figure out the cells along the certain path? Because I want to plot the heatmap for a certain path.

Thank you very much!
KC