Cna't figure out how the C++ Map container is populated in the IsIsogram()?

[Dick_Sram]

'm totally confused with the IsIsogram function of Section 2, Lecture 48, how did the hidden word get placed into the TMap variable LetterSeen?

Below, I commented the code in how I think it works

bool FBullCowGame::IsIsogram(FString Word) const
{
if (Word.length() <= 1 ) // If the number of characters are 1 or
return true; // less return true because it’s an IsIsogram.

TMap<char, bool> LetterSeen; //Create an empty map called LetterSeen,

for (auto Letter : Word)     // Take the characters in the string "Word
{                            // and place then in the string "Letter".

	Letter = tolower(Letter); //Convert the all the characters of "Letter" to lowercase.

	if (LetterSeen[Letter])   //HOW DOES LETTERSEEN GET POPULATED WITH CHARACTERS FROM "WORD"?
		return false;
	else
		LetterSeen[Letter] = true;
}	
return true;

}