Updated March 2017 (second paragraph added).

The W switch displays in wide (132) column mode (if terminal supports it). This allows the display of additional job status information, including the number of disk reads, writes, queue locking operations, commands (or programs) executed, BASIC instructions executed, keystrokes entered, and accumulated time logged in.

The job activity statistics are maintained in the jobtbl.sys file, making them available to be queried by utilities such as SYSTAT/W and JSTAT. Each job updates its own statistics in response to various triggers, the most universal of which is the Inter Job Communication clock, whose frequency is set by the IJCFREQ=<seconds> setting in miame.ini. Under UNIX systems, jobs can send a request message to other jobs using ITC.SBR, asking them to update their statistics immediately so as to be able to report the very latest. In order for such messages to be delivered, the jobs have to share the same effective user, or the sender has to be root. Failing that, the stats reported may be between 0-<IJC frequency> seconds old; you may also see an ITC error message displayed. Refer to Advanced Privilege Settings for more information.