Custom collections works on the premise of saving a combination of filters, view type and sort just like the current collections available to you:
- Home: All cards except for invisible
- Thoughts: Cards without a title or content or a parent
- Tasks: Cards containing at least one unchecked to do item
They are separate from cards and you will not able to share collections (at least not initially). So @JohnCP defining a new parent card for a collection will not work. And @DunkinDonuts the link to reminders is correct and what you are looking for – reminders are unrelated to custom collections.