A custom field is one that, at the time of signing, is automatically populated with data that is specific
to the signer. For instance, a signer might have an Agent Number, and that number may need to appear in the
final document next to their signature. By using a custom field, the signer can specify their Agent Number,
and that number will automatically appear in any other field that has been configured to receive it.

More info: https://community.onespan.com/documentation/onespan-sign/guides/admin-guides/user/custom-fields