With Lync Online, DNS records are slightly more complex, consisting of two SRV and two CNAME records.
The first of these SRV records enables SIP federation so that your SIP domain can federate to external domains and to public instant message environments, such as Skype.
The second is for coordinating information flow between clients. There are then two CNAME aliases, one for redirecting the Lync online client sign-in, the other for redirecting Lync online mobile client sign-in