Look at ProductDetail rather than Rule, as it's a little easier to understand, initially:

concept ProductDetails {
   o String productType
   o Integer quantity
   o Double pricePerUnit
}

Concepts are minor, but helpful elements, in the model. They are neither assets nor participants–they merely define the structural elements contained within them.

This preceding concept defines ProductDetail. We might argue that this is in fact an asset–for the purposes of our application, it's not something that gets transferred between participants! It's maybe a little clearer when we look at the Rule concept, which captures the terms and conditions of the letter of credit:

concept Rule {
   o String ruleId
   o String ruleText
}

This is something that is less like an asset or a participant, but it is helpful to have as a separate type, as it reveals an important structure.