Collections and messages are described in the chapter "More Advanced Schema and Instance Features". The relationships in the schema are filtered by the message used when the instance is called. The default message is 'create', so if one isn't specifically added to the called URI, #create is appended.
So in your example of the profile instance, it's generally called using a specific message, for example:
All of the schema fields that specify this message will be called in the order that they appear in the schema. In the case of the profile instance only one entry has that message, so that's all that gets run.
collect column propagates an attribute defined in a called method up to the caller, or to $evm.root.