Core features of XMPP are defined by RFC's, which can be found at IETF.org
Extended features of XMPP are defined by XEP's, which can be found at XMPP.org/extensions
XMPP is an open standard, rather than open software (it includes plenty of open software as well)
The XMPP standard is approved by...
How would one know certain XMPP extensions are enabled? or how would they be enabled? For security, for cool features, and for purposes of reducing overhead on constrained networks.
Core specifications carry the same XEP number throughout their entire cycle. Final and Draft extension protocols...
How is SIMPLE for an XMPP messaging alternative?
Both are accepted IETF open-source standards.
XMPP is great, except it has a lot of overhead (sometimes heavy lag) because it uses XML. Is SIMPLE as secure as XMPP?