Satellite channels

Yes, @DR0719, most channels from the receiver can be streamed on the DIRECTV App, but there are exceptions due to separate carriage agreements, such as some sporting events and Public Interest Channels. Complete details on the channels available via the App can be found here: DIRECTV-STREAM-Channel-Lineup.pdf John, DIRECTV Community Team

So does this mean if the channel isn’t on Directv stream, like Flix or the Movie Channel, they are not available for streaming ?

Right. At least not for streaming via DirecTV. You'll need to subscribe to the network's stream. 

DirecTV (satellite) and DIRECTV STREAM / DirecTV via Internet (streaming-only) don't have the exact same channels. View them as separate TV providers.

Not all channels for the satellite service are available to stream. This service was built with streaming as a secondary perk. Main service is for the home, paying for each authorized TV, etc.

If you want to stream all channels in your service, than you need a streaming service to begin with.