I guess, per 6e- personas are the specific type of icon that a person has.
The combat knife, under normal circumstances, has a little knife icon. If someone is using it to make a call (Perhaps with trodes hooked up, or perhaps that's not necessary), then the person using the knife has a persona- so the knife's icon is replaced with a persona.
It would probably be a crappy persona- there are rules for if a device lacks attributes.
So, unless you're using the knife as your commlink, it doesn't have one. But if you are, then it does have one.
I could also be completely off per fiction