The printer mirror iconbar icon is displayed when PsiFS is copying all characters received over the serial link to the printer. This allows a SIBO or EPOC device to print to a parallel printer without needing a special cable.
The text under the icon changes to indicate the number of bytes received over the serial link. This can be useful to confirm that the printer mirror is operating as expected.
More information is available on enabling and disabling the printer mirror. This includes details specific to printing from SIBO and EPOC devices.