Are alpha characters used for field names valid in MARC records?

When we do dumps of MARC records our ILS often dumps them with FMT and  
CAT field names. So not only do I have glorious 246 fields and 100  
fields but I also have CAT fields and FMT fields. Are these features  
of my ILS -- extensions of the standard -- or really a part of MARC?  
Moreover, does something like Marc4J or MARC::Batch and friends deal  
with these alpha field names correctly?

-- 
Eric Lease Morgan