Question

What is KLMN?

Answer 1

The KLMN notations indicate the total number of electrons with each principal quantum number which is $n$. The electrons can be distributed in the KLMN based electron shell. The $K$ shell is the first shell or energy level, $L$ is the second shell, $M$ is third, and so on.

Complete answer:
The names of the electron shells were given by a spectroscopist named Charles G Barkla. He named the innermost shell has k shell because he noticed that the X-rays emitted two types of energies. These energies were named as type $A$ that is higher energy X-ray and type $B$ that is lower energy X-ray.
The KLMN method is based on electron shells, with the labels KLMN being derived from an experiment in which the spectroscopist wanted to leave room for lower energy transitions in case there were any. $K$ denotes the first shell or energy level, $L$ the second shell, $M$, the third shell, and so on.
KLMN represent energy shells given by Bohr theory and represented by principal quantum number $SPDF$ represent orbitals within each of the shells given by sommerfield and represented by azimuthal quantum number Note that n can take any natural number value while l can take values from
$0$ to $(n - 1)$.

Note:
The difference between the KLMN and SPDF is $K$ denotes the first shell or energy level, $L$ the second shell, $M$, the third shell, and so on. In other words, the KLMN notation only indicates the number of electrons an atom has with each principal quantum number. The SPDF notation subdivides each shell into its subshells.