In Qing Liu's book Algebraic geometry and arithmetic curves I came across several confusing definitions. Several times he defines a notion only for a subclass of schemes/morphisms but later he is never explicitly mentioning these extra conditions again. Here are some examples:
Let $X$ be a locally Noetherian scheme, and let $x \in X$ be a point. We say that $X$ is regular at $x$ if [...]. We say that $X$ is regular if it is regular at all of its points. Question: If he later says "Let $X$ be a regular scheme", then is it implicit that $X$ is locally Noetherian? If so, then why doesn't he say "A scheme is called regular if it is locally Noetherian and [...]"?
Let $X$ be a reduced Noetherian scheme. Let $\xi_1,\ldots,\xi_n$ be the generic points of $X$. We say that a morphism of finite type $f:Z \rightarrow X$ is a birational morphism if [...]. Question: If he later says that a morphism $f:Z \rightarrow X$ of (arbitrary) schemes is birational, then is it implicit that $X$ is reduced Noetherian and that $f$ is of finite type? If so, then why doesn't he say "A morphism $f$ is called birational if it is of finite type, if $X$ is reduced Noetherian and if [...]"?
Now it gets really confusing: Let $X$ be a reduced locally Noetherian scheme. A proper birational morphism $\pi:Z \rightarrow X$ with $Z$ regular is called a desingularization of $X$. Question: He defined birational only for reduced Noetherian schemes. What is birational for reduced locally Noetherian schemes? Is his desingularization now automatically of finite type?
In Liu's book I found the following definition now: We say that a morphism $f:X \rightarrow Y$ is proper if it is of finite type, separated and universally closed. So, first of all, I think that this definition is now given in the non-confusing style, and second, this implies that the desingularizations above are of finite type (although it doesn't answer the locally Noetherian/Noetherian question).
I was asking "...then why doesn't he say that..." because I wasn't sure (and I'm still not sure) if there is some "higher truth" in this style of definition. Of course nobody except for Liu himself can answer this but perhaps someone else has more experience than I have and can give an explanation for this...