RFC 3312, section 11:
Option Tag for Preconditions
We define the option tag "precondition" for use in the Require and
Supported header fields. An offerer MUST include this tag in the
Require header field if the offer contains one or more "mandatory"
strength-tags. If all the strength-tags in the description are
"optional" or "none", the offerer MUST include this tag in either a
Supported header field or in a Require header field. It is, however,
RECOMMENDED that the Supported header field be used in this case.
The lack of preconditions in the answer would indicate that the
answerer did not support this extension.
The mapping of offers and answers to SIP requests and responses is
performed following the rules given in . Therefore, a user agent
including preconditions in the SDP MUST support the PRACK and UPDATE
methods. Consequently, it MUST include the "100rel"  tag in the
Supported header field and SHOULD include an Allow header field with
the "UPDATE" tag .