What is the difference between o and -- in Fabric Composer

Question

Look at the two versions of this code:

asset Document identified by docId {
    o String docId
    o Articles[] articles
}

asset Document identified by docId {
    o String docId
    --> Articles[] articles
}

Are both correct? If yes, when should we use what?

Answer 1

'o' indicates has-a relationship

'-->' indicates pass by reference