At Distributed Proofreaders, a character suite is a pre-defined group of characters that we use for working with our texts.
We use character suites because the full unicode standard includes a vast set of characters, and we only want a small subset of them for preparing our books. We can give some flexibility, by providing different character suites for different needs, but still constrain the characters available to use within reasonable "guiderails".
See All Character Suites for a list of all character suites installed on our system, both enabled and disabled. Disabled character suites are still under development.
The proofreading interface restricts the characters we use in the rounds to only those in the character suites (and custom characters) assigned by the Project Manager. Any characters entered outside of that will result in an error message when trying to save a page.
Usually proofreaders don't really need to worry about character suites. You can mostly just type characters you need with a keyboard, and use the character picker to access any others. Most of the texts we work with already have all the characters you need available in the default Basic Latin.
If a project manager has added a character suite other than Basic Latin for a particular book, you will see pickersets with additional characters in the proofing interface.
If you do run into something unusual, and have no instructions for how to handle it in the project comments, please see the Accented, Diacritical, and Non-ASCII Characters section of the Proofreading Guidelines.
When creating a project it would be ideal to be aware what characters are present in it, and evaluate what character suites are appropriate for it.
See Character suites for Project Managers for more information.