In the *specific* example of Flynn, there was no "unmasking". I'll explain why.
Flynn's communication was picked up because he was under FISA authorized surveillance.
#1 Incidental collection = unmasking request.
#2 Direct intercept / *Legal* = Active FISA Title-1 surveillance authority.
#3 Direct intercept / *Illegal* = Active surveillance without Title-1 authority.
A direct and legal intercept via active FISA Title-1 surveillance authority.
Flynn was carrying a surveillance ebola virus. He was unwittingly infecting everyone he communicated with.
However, the FISA warrant itself reconciles all of the issues around Flynn "unmasking", .....because there wasn't any.