X replies are any multiple of 4 bytes with a minimum of 32 bytes. The X server will forward to the X client an event that the application is expecting. This could include keyboard or mouse input. To minimize network traffic, only expected events are sent to X clients. The X server will report errors in requests to the X client.
Errors are like an event but are handled differently.
X errors are the same size as events to simplify their handling. They are sent to the error handling routine of the X client.