I don't necessarily have an official policy on reserved but generally if someone wants a custom order, I will make them a 'reserved' listing. These are special requests and they will be created only when someone asks (and states they are ready to purchase) and only stay in my store for a short period of time.
This is different than having a 'custom' listing where I have a general item in a pre-packaged format that someone can purchase an item that I will make custom for them (for me, it's a logo package).
Sometimes people don't want what is in my custom packages or want to substitute items, so that's when I make a reserved listing.
I think it really depends on what you sell and how you sell it. If you allow people to contact you for custom items then it would make sense if they request a custom order that you make them a reserved listing.
When that happens, I will just put "RESERVED" in the title and "Reserved for XX" (initials for privacy) in the description.