An expression is a customer message that has been trained to a specific intent. By assigning a message to an intent through training, the message becomes an expression under that intent.
A good expression is a message that represents the intent topic(s) well. Expressions can be short and concise, long and detailed or anything in between. An intent should be trained with a mixture of expressions that reflect the data and customer behavior.
The same expression or similar expressions should never be trained to different intents. This will cause considerable confusion for the AI model, and for an intent to be incorrectly triggered and trigger other intents incorrectly.