"I can start songs on key"

In this context, does "on key" mean "on the nearest comfortable key in concert pitch" or "pitched comfortably for singing?"

For unaccompanied singing, concert pitch is irrelevant or even, if an instrumental accompaniment is not wanted, undesirable. (To add to the rules, "no twangling of guitar strings, or pokes on the melodeon, to work out what key the singer is using.")