These (days, weeks, months?) I'm extremelly busy, but I though this is quite interesting. Deploying InfoPath forms with columns promoted programmatically, I found that the documentation is quite poor. So here it is:
If you add to a Content Type a Field with an aggregation function, the options would be:
plaintext Converts node text content into plain text.
first Applies property promotion and demotion to the first element in the collection.
last Applies property promotion and demotion to the last element in the collection.