Question: Where is there a full explanation of specfunc?

Below are five subsindets commands.

I believe I understand the actions of B and C, but I fail to understand the actions, individually and taken together, of  E, F and G.

B := subsindets(u(i, j)^2*v(i, j)+u(i-1, j), 'specfunc(u)', proc (f) options operator, arrow; subsop(1 = op(1, f)+1, f) end proc);

C := subsindets(u(i, j)^2*v(i, j)+u(i-1, j), 'specfunc(anything, u)', proc (f) options operator, arrow; subsop(1 = op(1, f)+1, f) end proc);

E := subsindets(u(i, j)^2*v(i, j)+u(i-1, j), 'specfunc(symbol, u)', proc (f) options operator, arrow; subsop(1 = op(1, f)+1, f) end proc);

F := subsindets(u(i, j)^2*v(i, j)+u(i-1, j), 'specfunc(`+`, u)', proc (f) options operator, arrow; subsop(1 = op(1, f)+1, f) end proc);

G := subsindets(u(i, j)^2*v(i, j)+u(i-1, j), 'specfunc({`+`, symbol}, u)', proc (f) options operator, arrow; subsop(1 = op(1, f)+1, f) end proc);

Where can I find a thorough explanation of specfunc with examples?