[Kwant] Difference between hop[0].tag and hop[1].tag

Dear users, Good day. My query is about hoppings. In fact, sometimes we have to change the onsite parameters when for instance a magnetic field is applied. What is the difference between hop[0].tag and hop[1].tag My second question the onsite fuction for example: def onsite(site, V):return V why it depends on site where the shape functions for example def circle(pos): rsq = pos[0] ** 2 + pos[1] ** 2 depends on pos Best A BELAYADI