Not quite sure what you are asking so I may be off base but here goes:
The track and clip envelopes achieve the same result.
If your track is one continuous clip the either will work.
If your track contains several clips then highlight the clip of interest and use a clip envelope on just that clip.
If you have several clips in one track and you choose the track envelope … then one continuous volume line will run the length of all the clips.
In either case you just insert nodes to control the volume.
So, if your track is continuous, the track envelope may be more convenient but if you have several clips in a track the it may be easier and more efficient to use clip envelope.
Please ignore if this is not what you are asking.