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Is the meaning of ‘all’ here ‘completely’ or ‘it=all’?

What do you native English speakers think? Thank you so much as usual in advance.

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Hans51Is the meaning of ‘all’ here ‘completely’

Yes.

Hans51or ‘it=all’?

I don't understand what you mean by that.

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Hans51‘it=all’?

I'm not certain what you are getting at here, but if you are asking whether "It's all about you" can mean "There is nothing more/further about you" then this is unlikely and potentially hard to understand. For this case we would generally say "It's / That's / This is all (that) there is about you", though "That's all about you" would also be clearer for this meaning (in the right context).

Hans51Is the meaning of ‘all’ here ‘completely’

Yes. Most literally, the sentence means that every part of it is concerning you. E.g. "Is some part of that letter about my friend?" -- "No, it's all about you". Colloquially, "all about" can be used slightly less literally, e.g. the point of saying "This book is all about whales" is probably not to literally emphasise that there is zero mention of anything but whales, but more to express that whales are the book's subject, and there is a lot of information about them in the book.

"It’s all about you" can refer to information subject, as in the "letter" example above, or in another kind of usage it can imply that the person thinks that they are the centre of everything. E.g. "You never think of others, do you? It's all about you".