You can specify pretty much anything that your CA allows.
The relevant RFC is RFC5280. It says in section 126.96.36.199. "Subject Alternative Name"
The subject alternative name extension allows identities to be bound
to the subject of the certificate. These identities may be included
in addition to or in place of the identity in the subject field of
the certificate. Defined options include an Internet electronic mail
address, a DNS name, an IP address, and a Uniform Resource Identifier
(URI). Other options exist, including completely local definitions.
Multiple name forms, and multiple instances of each name form, MAY be
included. Whenever such identities are to be bound into a
certificate, the subject alternative name (or issuer alternative
name) extension MUST be used; however, a DNS name MAY also be
represented in the subject field using the domainComponent attribute
as described in Section 188.8.131.52. Note that where such names are
represented in the subject field implementations are not required to
convert them into DNS names.
You should read the rest of that section, and then check with your CA what they support. It's worth noting that your CA must verify that all subject alternative names are correct.
To use an email address, the RFC says in section 184.108.40.206
Conforming implementations generating new certificates with
electronic mail addresses MUST use the rfc822Name in the subject
alternative name extension (Section 220.127.116.11) to describe such
identities. Simultaneous inclusion of the emailAddress attribute in
the subject distinguished name to support legacy implementations is
deprecated but permitted.
So instead of UPI, you should use rfc822Name.