The limitations to using custom fields are as follows:
The maximum size of a custom field file cannot exceed 500 KB
The title you provide to a custom field cannot exceed 256 characters
You can install maximum 50 extensions (including custom fields and custom widgets) in a stack
The maximum size of the configuration file of a custom field cannot exceed 10 KB
The maximum size of a custom field file that uses JSON Data Type cannot exceed 10,000 bytes (10KB). This limit is configurable. For more information, contact our Support team.
The maximum number of JSON type custom fields that can be added per content type is 10. This limit is configurable. For more information, contact our Support team.
JSON object key(s) cannot start with a dollar '$' sign or contain a dot '.' character.
Internally as well externally hosted custom fields are not supported on Internet Explorer and Opera Mini as these browsers lack support for the <iframe> srcdoc attribute. Read more.
Extensions are loaded in an iframe in sandbox mode for security reason. Hence, the cookies are not exposed. However, as popups are allowed in the current sandbox mode, you can open the popup window and access the cookies or storage in the popup window.