Line holder or reserve, it doesn't matter. I believe the preferred method by ANY employer is to let the company know your Drill dates as soon as you can. At SkyWest I'm sure they would appreciate knowing them prior to the PBS load so they could pre assign you for it. My unit published our UTA's for the entire year. Sometimes these were subject to change, but I could usually let skywest know ahead of the month so it wasn't an issue.
as for AFTP's or any other status less than 30 days, you simply need to notify scheduling that you will need MIL days on these dates. If it happens to be in the middle of a trip you can use user time to cover the legs lost to get you back to domicile. You are not required to provide orders. They may ask, but it is not required unless you go over 30 days.