I came across a code with a different format that I am accustomed to used for Function and its format. The original post is here and also in here

SphericalBesselI[0, 0] := SphericalBesselI[0, 0] = 1;
SphericalBesselI[l_, z_] := Sqrt[\[Pi]/(2 z)] BesselI[l + 1/2, z];
(*expansion coefficients for the Henyey-Greenstein phase function*)
\
\[Sigma][l_][g_] := g^l
(*diagonal matrix elements*)

h[l_][g_, \[Omega]_] := 
  h[l][g, \[Omega]] = (2 l + 1) (1 - \[Omega] \[Sigma][l][g]);

FIRST LINE--- What is the purpose to use the build in function as the function name and then equal to one?This is confusing as I thought the function shall start with small cap and the equal to the right if further confusing.

FIFTH LINE: What is the meaning of having 2 square brackets in the function definition?\[Sigma][l_][g_] := g^l