Allow me to ask four variants of this question in order to try and make clear what the point is. I will also answer each of them according to my current understanding, which I seek to find out if is right or not.
Suppose that I obtain a timestamp (through a public agency, a notary etc.) for the document describing an invention, and then I do not reveal it publicly.
(i) If at a later time I apply for a patent, will this action affect the application's fate?
My current guess: No, since it wasn't revealed.
(ii) If at a later time someone else either applies for a patent or uses something quite similar to the invention, can I use this action to protect against it?
My guess: No, because there's no prior art established.
Now suppose that I obtain a timestamp for the document describing an invention, and then I deposit it at an appropriate depository.
(iii) If at a later time I apply for a patent, will this action affect the application's fate?
My current guess: Yes, negatively, since it was revealed.
(iv) If at a later time someone else either applies for a patent or uses something quite similar to the invention, can I use this action to protect against it?
My guess: Ehm, no clue.
Thanks for any clarification that you can give on these points.
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Note: This question is a hopefully improved version of this one; I believe that it is not a duplicate, since I have understood more in the meantime and added relevant detail.