What is the purpose of a custom door?
You may need to have a custom-made door for any number of reasons; special needs or disability access requirements, a grand entranceway for your stately home, a converted window casing or an unusual opening such as an archway. All of these situations are likely to require you to have doors specifically constructed for installation.
What kinds of things can be customised?
Virtually any part of a door can be tailored to match your design requirements, as long as the door remains stable and is able to open and close. The materials the door is constructed from, the shape, height, width, the door furniture and accessories can all be selected according to your needs. You may want a special lock system for added security, or to fit customised dutch doors in a kitchen or a cat flap on the back door. Or triple glazing or special window treatments, if you want glass doors.
Doors of different shapes can also be installed, but you may wish to consider how these will affect both your ability to get through the door, and the door's strength. A star-shaped door might make you feel pretty special, but it's also likely to be considerably weaker around the edges than a conventionally constructed door.
What you may need to consider when choosing a custom door
Aside from the aesthetics of a custom door, you will need to think about the practical aspects too. The size and weight of the door will determine how practical it is and how easily it opens, as well as how much strain the hinges will be under. A heavy door will place a greater load not just on the door hinges though - it'll also affect the door frame, and even the wall that's attached to.
If you're planning from scratch this isn't a huge problem as the weight of the materials will be factored into the overall engineering of the house's structure, but if you're looking at retrofitting an unusually large or heavy door you may wish to run your plans past a structural engineer to make sure it's being attached safely. The supplier of the door may also be able to advise on what's required.
Size and access
It may seem obvious, but you'll also need to consider how it opens and closes. Having a big hinged door that opens outwards might seem like a good idea, but if it's taking up most of an otherwise narrow entryway it's likely to mean that people have to shuffle back out of the way just to get in the house. By the same token, if it opens inwards you'll need to forfeit a large amount of space to accommodate it when it's fully open.
Hinged doors always need a small amount of clearance between the door and the jamb to compensate for the thickness of the door as it opens. The thicker your door, the more you'll need to account for this and consider how it may be allowing heat through.