You can create as many of these as you need. I use one for OAC and another for Poly Sci mail. Note that you can add features to these signatures, but people need a graphical mail package to read them. You can even scan your real signature if you so desire. I have added special fonts and a Bart Simpson graphic to my signature for this mailing as an example.
Many people use only one signature and it is by definition and name the default.
Switching between signatures is easy. The same principle applies to stationery. Stationery is for sending the same message or headers to the same people. I use it as a form. It can include all fields and even parts or all of a message.