Schemes in DealTrack are extremely versatile, and the scheme lead is just another characteristic that can be recorded against a scheme.
In DealTrack, each scheme with automatic approval workflow will have a scheme lead marked against it. The scheme lead will be the user who has started the approval workflow process. This will be carried through to the active schemes tab once the scheme has been fully approved. This is so you can track who decided the scheme was ready to go through the whole approval process.
Please note that only users in the workflow roles that are configured to be able to start workflow are able to be scheme leads.