Is `image:` a global keyword?

I am new to gitlab, and please pardon me for asking rudimentary questions.
I have watching a lot of tutorial videos and read the reference. I see image: used as a global keyword in almost all tutorials. But in the reference, it is not a global keyword. So I am a bit confused. I guess a toplevel image: is the same as default: image:, but I haven’t found a documentation to confirm. Can you please clarify this for me? Thanks.