This is the security Account Name which the AF Server is running under. This setting can be configured on the Client side, but normally it stays blank unless you have the error, see https://techsupport.osisoft.com/Troubleshooting/KB/KB00359.
The GUI version of the accountName setting is here:
Thanks Anna for the information.
It was just for my curiosity.
I put it back to blank for one of our setup because it avoids the Kerberos delegation to work properly (if an account is specifiedon it).
The KB you have provided explains what I need to know (especially at the end: "Below is the release note for PI AF 2.2 regarding the fix, WI 6614:...)