The One Contract Per Release setting is used to set up multiple Contracts per track/release (our overview on Contract Ins can be found here).
If this is set to No, you will only be able to add a track or release to a single Contract In. The track will not show when searching in the release search.
If this is set to Yes, you will get the following warning when adding an existing track:
Press the tick box & then select Create, you will then have the second Contract In for the track.