public String getSubjectId() {
        return subjectId;
    }
<p>If this is a user then {@code subjectId} is also the unique identifier for the user on the
OpenIdConnect IDP, i.e. the subject. The value may be a UUID or a more human friendly form
depending on the IDP in use (internal/external).</p>

<p>If {@code isGroup} is {@code true} then this is the unique identifier of the group.
A group identifier is defined by the user so is likely to be human friendly.
A user and a group can share the same name.</p>

@return The unique identifier for this user or group.
stroom-core-shared/src/main/java/stroom/security/shared/User.java