How does a person go about exporting MARC records from a III system?

As you may or may not know, I spend a lot of my time developing a thing colloquially called  the "Catholic Portal". It uses VUFind under the hood, and it requires me to ingest bibliographic data from a myriad of libraries.

Suppose the records I desire have the letters "crra" saved in MARC field 590$a. What is the process for connecting a III system, searching for "crra" in 590$a, and saving the result as a file of MARC records? Is there some sort of documentation I can read that will help me out in this regard?

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Eric Lease Morgan