The lead project is the one that's moving ahead in development, while the backup task is lagging (or working in sync) to provide an alternative asset should the lead project fail. In other words, if your lead project crashes and burns, you've got a backup ready to go that can pick up the slack. Backups usually are very similar to the lead project in terms of mechanism of action and performance. Otherwise, they wouldn't be much use as a replacement. But, of course, having two very similar products on the market would be a disaster, so companies usually only move one of them forward to commercialization. Better safe than sorry, right?