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Context is important here.
If this special message is from doing something unusual, I'd file it under Easter Egg. "Congratulations! You found the secret!"
If there's a whole room of messages or developers speaking to you, that's Developer's Room.
If it's something along the lines of "Thanks for playing our game", that's Thanking the Viewer.
If it's outside the scope of the game — for example, a Concept Art Gallery with commentary — I'm not sure that needs any further distinction. Some galleries might inform the player about things, and some might not.
Well, in actuality here's the context: You're about to stumble upon what would be an unexpected boss battle had the creator not warned you about it and healed up your team in advance just as you would go to the battle spot. (Pokémon Radical Red)
I have things to do and daddy issues to repress!
A Healing Checkpoint, and also Suspicious Videogame Generosity bordering on a variant of Ominous Save Prompt- that is, the fact that you're being explicitly warned and healed for free here should be taken as a hint that this is going to be one of those battles.
Edited by Scorpion451
It isn't necessarily Breaking the Fourth Wall; it can be merely a small message box suddenly popping up "from the creator himself" to the person playing the very game as they stumble upon a certain checkpoint.
Edited by Monolaf317