To the left of the main key area is a field where you can add an external curve asset from the Content Browser to that track. This is different than using the button, which creates a new
track from a selected external curve.
External Curve Icon
Use the curve selected in the Content Browser for this track.
Browse in the Content Browser to select a curve for this track.
Convert an imported external curve to an internal curve so that keys and curves can be edited.
However, you may also Right-click on any curve (with at least one key selected) and choose Create External Curve. This brings up a context window in which you can edit the path into which the curve asset will be saved. In doing this, you may then use this curve again on other timelines.