sel is short for "selected number of atoms in rcut".
sel_a[i] is a list of integers. The length of the list should be the same as the number of atom types in the system.
sel_a[i] gives the number of the selected number of type i neighbors within rcut. To ensure that the results are strictly accurate, sel_a[i] should be larger than the largest number of type i neighbors in the rcut.
However, the computation overhead increases with sel_a[i], therefore, sel_a[i] should be as small as possible.
The setting of sel_a[i] should balance the above two considerations.