Adding comments would clarify

A := STD.Str.Contains(
'the quick brown fox jumps over the lazy dog',
'ABCdefghijklmnopqrstuvwxyz', true); //returns TRUE

B:= STD.Str.Contains(
'the speedy ochre vixen leapt over the indolent retriever',
'abcdefghijklmnopqrstuvwxyz', false); //returns FALSE – 'z' is missing