13.44A melody can determine which extension to choose

Suppose you're on G7.

Melody = A.

A is 9.

So:

G9

is natural.

Melody = E.

E is 13.

So:

G13

makes sense.

Melody = A♭.

That's ♭9.

So:

G7♭9

may describe the sound.

The melody often tells you the chord extension automatically.