dear friend its just a method of representing the various orbitals. actually it is the azimuthal quantum number for l=0 we have s orbital l=1 p orbital l=2 d orbital l=3 f orbital and so on similarly principal quantum number is represented by K,L,M etc also you can see that PQN(principal quantum number) is represented by capital letter so for convenience AQN(azimuthal quantum number) is given small letter
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