To test different subject lines with a pilot group and then send the winning subject line out to the remaining customers, you should place a random split before path optimizer for the pilot group. This will allow you to divide your audience into two groups: one for testing and one for holding back. Then, you can use path optimizer to test up to 10 subject lines with the testing group and select the winner based on open rate or click rate. Finally, you can send the winning subject line to the holdback group.