Google will allow 3rd party developers to implement its Google Assistant anywhere, but not yet
Direct is used for when the user's request is straightforward and gives enough information. For example “Hallway lights on”. A Conversation Action, on the other hand, is one that might require the Assistant (or the 3rd party service's own bot) to ask for some clarification. As an example, Google showed us how calling for an Uber ride via the Google Assistant would work. First, the user asks the Assistant for an Uber, then they are connected with the latter's own bot. The Uber bot will ask where you'd like to go and if you'd need a bigger car, for example.
The Google Assistant partners, currently hard at work integrating it in their services
Things that are NOT allowed: