What is the difference between the Comprehension scheme an Replacemnt scheme?

Question

I am currently studying ZFC axioms and having trouble understanding the difference between the comprehension scheme and the replacement scheme. Here's what I understand about the two schemes so far. The comprehension scheme allows us to create a new set whose elements are members of a given set obeying some property and the replacement scheme let us create a new set which is the image of an existing set under a function. So both the scheme just creates a new set with some elements from already existing set according to some property P.

Answer 1

Comprehension requires that the elements already be part of a set. You are making a subset from the elements that have the property of interest. Replacement allows the elements of the new set to come from anywhere as the range of the function may not be known to be a set. If I apply comprehension to $\Bbb N$ I can get any subset of the naturals that I can describe. If I apply replacement to $\Bbb N$ I can get any countable set that I can describe.