A leftmost derivation of a string in the language defined by a
grammar $G$ characterizes a parse-tree for that string. It is a way of
describing the parse tree, which is unique for each parse tree.
But to answer the specifics of your question, there is no lefmost
derivation of a language, only of the strings of a
language. Furthermore, since a leftmost derivation is a sequence of
grammar rules applications to produce the string, a leftmost
derivation is necessarily defined with respect to a specific
grammar. Indeed, the language does not really matter, only the grammar
and a string derived with that grammar.
If a string has only a single derivation for a grammar $G$, it is said
unambiguous for that grammar. When it has several, it is said
ambiguous for that grammar. When it has none, it does not belong to
the language generated by the grammar.
A grammar is ambiguous if it generate at least one ambiguous
string. It is unambiguous if every string in its language is
The grammar in your question is unambiguous. Each of the derivations
you give is a leftmost derivation, but each for a different string in
the language of the grammar.
Actually, this grammar is so simple that it has only leftmost
derivations, because there is at most one non-terminal in each rule
right-hand side. But a grammar can have only leftmost derivations and be ambiguous.