[Koha] Newbie: MARC Framework editing question

Hi, Please bear with my ignorance ... I have created a noe MARC framework (BOOKS) and also an authorized value "Languages". The framework is derived from the Default framework and accordingly has a field "041 - LANGUAGE CODE". I proceeded to have that mandatory and linked it to the created authorized value. My question now is: how does this actually work when importing something via z39.50 from let's say the LOC ... it's possible that MARC field definitions from souces conflict with my local setup. How are conflicts handled? Is it safer to generate a new (local) field rather than editing like done above? Thanks for any hints. Sincerely, Joh