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SSequence said:(b) Suppose we are given some arbitrary model ##M##. In some specific cases it may be the case that all the ordinals in ##M## are also in ##Ord## (class of ordinals). However, it is worth noting that there may be cases where this does not hold [if we have a ##V## with standard ##\mathbb{N}## then I think perhaps an example of this could be an ##M## with non-standard ##\mathbb{N}##].
I must admit that when ordinals are brought up the topic usually moves beyond me. The only thing I "know" is that given the Von Neumann encoding, a number is the "set of all lower numbers" (e.g. ##2 := \{\emptyset, \{\emptyset \}\}=\{0,1\}##) so then we can define infinite ordinals ##\omega:=\mathbb{N},\, \omega+1=\mathbb{N}\cup\{\mathbb{N}\},...## and then go from there. Beyond that.. nada.