Not even counting conflicts for travel, at a minimum every pilot on 12-13 days off must have training put on their working days so they don't go below min days off. This requires the company to staff a certain number of extra pilots to cover those in training. Those extra pilots boost your relative seniority which controls when you can hold a domicile or upgrade. The company will no longer be required to staff those extra pilots if everyone including those with 12-13 day off lines now trains on days off.