Identity plays a major role in Shadowrun. Its a world in which everything you do is tracked and all the various megacorps are constantly profiling people. It can be difficult knowing who to give what name and how to act on the job.
Most often Shadowrunners will use their handles when interacting with other runners. It is unwise to give away too much info even with your teammates, after all, they may be the opposition the next job. Think of it as having a work contact number.
START OOC EXAMPLE 1
A group of runners are hired to perform an extraction. They want to meet before talking to the Johnson to make sure they are prepared so they exchange comm codes.
Lora Troy didn't want anyone to know her fake SIN, so she logged on as Patchwork and introduced herself as such. The others followed suit keeping their SINs secret.
END OOC EXAMPLE 1
Johnsons will USUALLY use the handles as well. The less they know about you, the easier it is to deny having hired you. This is not true in all cases though.
For instance a Johnson may have a legitimate job, something which is being recorded on the books. They will use whatever SIN you gave then out of good manners, they do not want to leave evidence of the connection between a runner's legitimate id and their handle.
START OOC EXAMPLE 2
Currently Marc Greenboro has hired runners twice. Both times he was working on behalf of NeoNet. The first time was on the books and the second was unofficial.
There was one runner who participated in both jobs, /usr/bin/grrl. In the legitimate job, she was addressed as Kyuusai.
Later on when she was hired again she did not realize it was the same Johnson and used a different ID. In that job, regardless of the ID she used, she was addressed as /usr/bin/grrl…
END OOC EXAMPLE 2
There are other exceptions, for instance while working for one Johnson the players reacted with surprise (rightly so) to how they were addressed. The Johnson was the type who wanted as much control as possible.
START OOC EXAMPLE 3
The Irish Mafia needed a job done. They had the names of some runners who were willing to do the job, but these runners didn't have enough street cred to be trusted. Mr. Green wanted to show them he was in charge and subtly put them in their place. He held the meet in a location which required them to broadcast their SINs and when they showed up he greeted them all by name.
The runners balked at this, but he was paying the bill and there was nothing they could do…
END OOC EXAMPLE 3