It: while we wanna refer to a thing,animal,idea, .... that has already been mentioned we use"it"
That: like "it" but while we wanna refer to a thing or idea or animal that isn't available we use "that"
Hey Tracey, did you get my files?
Sorry going through a busy time in office. Yes I analyzed the solution.
It will require some resource, which I could not manage.
Sending you some dough today, you can allocate the resources to this project.
Perfect. Thanks a lot!
I will have them ready by tonight.