onsgmls -wxml -s -f $(PREPDIR)$(NAMEPREFIX).errors $(XMLDECL) $(XMLDOCTYPE) $(XMLFILE)

Here is the command with the substitutions made for s/samplefa:.

onsgmls -wxml -s -f $DLXSROOT/prep/s/samplefa/samplefa.errors $DLXSROOT/misc/sgml/xml.dcl $DLXSROOT/prep/s/samplefa/samplefa.xml.inp     $DLXSROOT/obj/s/samplefa/samplefa.xml 

• -wxml tells onsgmls to output XML warnings (rather than SGML warnings)
• -s suppresses output. That means instead of getting a parse tree, we just get any errors
• -f ...errors tells onsgmls to write errors to a file with that name
• $DLXSROOT/misc/sgml/xml.dcl is an SGML Declaration for valid XML documents.
This lets onsgmls which is an SGML parser work with XML documents
• $DLXSROOT/prep/s/samplefa/samplefa.xml.inp is a document type declaration which points to the dlxsead2002.dtd. (See below). Its essentially the EAD 2002 DTD which allows the <ead> to be repeated and wraps the repeated <ead>s in <COLL> element tags.

Samplefa.xml.inp:

<!DOCTYPE COLL SYSTEM "/l1/release/13/misc/sgml/dlxsead2002.dtd" [
]>