Personal Certificates are very common in a PKI infrastructure. And I would say that most often it's stored on the computer of that user/person.
It would of course be preferable to have the certificate and private keys stored on a smart card as mentioned in another answer but there is limited support for smart cards in for example smart phones.
- One way to do this is to have unique keys and certificates (with the
same DN) on several devices. Then you can easily revoke a certificate
if a device is stolen. The problem with this approach is for example
encryption. If there was an email encrypted with the public key
corresponding to a certificate on your smart phone, that email cannot
be read on your laptop and vice versa.
- One middle ground would be to have different certificates on every
device but with the same private key, with this approach you can
still read encrypted messages on all units. But if one unit becomes
compromised you will need to revoke all certificates.
- And the simplest solution is to have the same certificate and keys
distributed to all your units. This have the same drawbacks as the
These certificates and keys are stored differently depending on OS and application. MacOS stores them in the key ring for most of the applications. Firefox can stored them in its own key ring, and so on.