There are two ways to deal with that.
When you want an asset, than you set asset
to required.
Then you use a feature called slotfilling. You set a prompt like:
What asset do you mean?
User says:
When will it fail
When will the unit fail
<-- mark the unit
as asset
In the Action segment you may set asset to required and set a prompt.
If it is set. API.ai tries to "fill the slot" by showing the user the prompt:
(Example: What asset do you mean?
)
If required isn't set:
When will it fail
will be accepted and the asset field is empty. In this case your backend has to handle it.
Either with a default value or an error message.
Of course it is better for the experience of the user, if he doesn't get a: I don't know what you mean
response ;-)