Below is the list of standard names for some of the loop variables:

l - index of orbital quantum number
m - index of azimutal quantum nuber
lm - combined index of (l,m) quantum numbers
ia - index of atom
ic - index of atom class
iat - index of atom type
ir - index of r-point
ig - index of G-vector
idxlo - index of local orbital
idxrf - index of radial function
xi - combined index of lm and idxrf (product of angular and radial functions)
ik - index of k-point
itp - index of (theta, phi) spherical angles
The _loc suffix is often added to the variables to indicate that they represent the local fraction of the elements assigned to a given MPI rank.