ixml/dtd_restriction is a SAP Parameter attribute which is used to control Restriction of DTD processing for iXML information. This is available within R/3 SAP systems depending on the version and release level.
Below is the standard documentation available and a few details of the attributes values .
A DTD can contain malicious code than can force an XML processor to
expand the resulting XML contend in a way to overload CPU usage and
memory consumption. This profile parameter is setting the default
processing restriction of iXML for DTDs to avoid a DoS attack by by
using malicious DTDs. Application programs can use the interface
IF_IXML_ISTREAM to to overwrite the default restriction.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.