A device is the SCA term for symbol of a person or group. It is more properly known as an escutcheon or shield of arms. This is what people paint onto their shields or sew into their clothing (or weave into their Chainmaille, etc). Devices are a big part of Heraldry.
The SCA has some standards for devices, and maintains a registry of charges. If you plan to register your device, you'd do well to consult a Herald before you get too bogged down in ideas. It's fine to work on the registration process on your own, but you will have to submit it to the College of Heralds for approval.
What makes a good device
- As few colours as possible
- As few charges as possible
- Every charge should be from pre-17th-century heraldry (sorry guys, no Loony Toons characters)
- A style that was used in pre-17th-century heraldry
- Repetition or mirror-imaging of a charge
- High contrast