Codeshare flight vs non code share flight
 

I am booking flights to HKG. Some of the ANA flights sold by United is under UA9XXX, but some other flight shows their original flight number e.g. NH811

What's the difference and logic behind?

UA doesn’t codeshare every flight, presumably because they don’t have an agreement in place with NH for each one. When UA has a codeshare, united.com will always use it. The only real differences are that they can have different inventory, meaning the fares can be cheaper or more expensive than the native codes, and you may have seat selection issues (I forget if NH is one of the carriers that has that problem). If you’re trying to upgrade, you’ll need native codes as well, but that’s a niche case. Mostly, you can ignore it.

if no price difference ... always better to take the native flight number. Makes it easier to request services and when making changes. As long as it is a 016 ticket you still have the same re-booking flexibility.

In the case of JV partners such as NH it generally doesn't hurt to have a codeshare #, but it doesn't help with anything either.

While that’s 100% true in the abstract, it’s often difficult / impossible to get a native flight number when UA has a codeshare. Google Flights will sometimes do it; an agent might do it as long as there’s a UA flight somewhere on the itinerary. And travel agents often have the ability to choose. But if you search on united.com, and there’s a UA codeshare available on the flight, I’ve never seen a native flight number offered instead.