Expect to get a couple of questions about when to use a certain parser (SAX, StAX, DOM, but also in combination with XSLT and even JAXB which is not part of JAXP).
You need to know the differences / similarities of the JAXP parsers and its best usage. You don't need in depth knowledge of the coding aspect. You might want to read
JSR-206 and the Oracle
JAXP tutorial