I am trying to draw a fin for my final year project, but I could not fully understand the technical drawing explanations that I marked with red markings.

especially R1.281 DEFINES TANGENCY OF R1.312 WITH FIN SURFACE, I couldn't understand the part.
My goal is to make a "bubble fin holder" like in the last picture.
I'm also wondering how we can draw this part of fin .
[![enter image description here][1]][1]


[![enter image description here][2]][2]



[![enter image description here][3]][3]


[![enter image description here][4]][4]


[![enter image description here][5]][5], 

[![enter image description here][6]][6]



  [1]: https://i.sstatic.net/X5Whl.jpg
  [2]: https://i.sstatic.net/Tjwyc.jpg
  [3]: https://i.sstatic.net/WIBOO.png
  [4]: https://i.sstatic.net/hvGbH.png
  [5]: https://i.sstatic.net/VcOcz.jpg
  [6]: https://i.sstatic.net/uIX6w.png