Improper style for OPLS-AA force field

I’m trying to create a LAMMPS datafile using the OPLS-AA force field for an Ionic Liquid by using the parameters from this paper.
The torsional part of this ff is given as:

The improper coefficients in the SI of the paper are given in this form:

How do I interpret the X in these “X-C/N-X-X” improper? and what improper style should I use for these kinds of improper?

X in this context usually means any atom type. The non-X atom type would be the central atom

You could either define them as dihedrals using dihedral style opls or use improper style cvff with d=-1 and n=2 and K=\frac{1}{2}V_2 since the other fourier coefficients are zero.

While writing the impropers in the datafile as “n improper_type I J K L”; would the atom I be CW/CR/NA?
Also, I there are 3 atoms (J,K,L) bonded to I, does the order of writing them?

Please carefully study the documentation for the improper style in question.